Originally Published: 24 MAR 22 00:07 ET

Updated: 24 MAR 22 09:48 ET

By Kevin Liptak, CNN

(CNN) -- Leaders attending a slew of emergency summits here Thursday were working to settle on the next phase of their response to Russia's war in Ukraine, with new US sanctions and refugee assistance among the steps emerging from the snap talks.

US President Joe Biden announced new punishment directed at members of Russia's parliament and unveiled a plan to accept as many as 100,000 refugees fleeing the violence in Ukraine, steps intended to show American resolve in confronting the crisis.

A discussion of NATO's force posture along its eastern edge was also part of the last-minute diplomatic burst. And leaders conferred on what to do if Russia deploys a chemical, biological or even nuclear weapon, a prospect causing increasing concern as the war reaches a stalemate.

As the last-minute NATO summit got underway, leaders heard a call for more help from Ukrainian President Volodymyr Zelensky, who addressed the gathering virtually. He stopped short of issuing his usual request for a no-fly zone. But he did say Ukraine needs fighter jets, tanks and better air defenses.

"You can give us 1% of all your planes. One percent of all your tanks. One percent!" he said in a virtual address to the summit.

Following Zelensky's remarks, a senior US official said the US is still opposed to providing fighter jets to Ukraine. Previously, US officials said providing such jets to Ukraine could be viewed by Russian President Vladimir Putin as an escalatory step.

Representatives from the White House and European governments spent the days leading up to the summit in intensive conversations finalizing steps to unveil following the talks.

Leaders including Biden arrived to NATO headquarters Thursday morning intent on demonstrating unity amid Russia's aggression. They posed for a brief family photo before entering the lengthy closed-door session. The mood inside the meeting was "sober, resolute and incredibly united," one senior US administration official said.

"We gather at a critical time for our security," NATO Secretary General Jens Stoltenberg said as the meeting convened. "We are determined to continue to impose costs on Russia to bring about an end to this brutal war."

"We are all doing more on land, at sea and in the air," he went on. "This is necessary to respond to the new security reality in Europe."

He said leaders would discuss ways to "strengthen our defenses now and for the years to come."

What they won't do, however, is what Zelensky has repeatedly asked: Enforce a no-fly zone over Ukraine. US and NATO officials have repeatedly said that such a move would risk provoking Putin and sparking a wider war with Russia. Western allies have also found it difficult to take more aggressive steps, such as providing Russian-made fighter jets to Ukraine or deciding to cut themselves off from Russian energy supplies, which could potentially cripple Russia's economy.

The crisis sessions of NATO, the European Council and the G7 were arranged last-minute, leaving little time for the normal back-and-forth between governments that precedes such events. Biden determined earlier this month that an in-person gathering of a newly united Western alliance would signal resolve to Putin.

Whether Putin views it that way -- or whether cracks are exposed among the allies on sanctions and use of military force -- remains to be seen.

Biden hoped to set a tone on sanctions by slapping restrictions on 300 members of the Russian Duma, the lower body of Parliament, and over 40 Russian defense companies, a senior administration official said.

"The overall message here is we have taken historic steps in imposing costs on Russia, now let's make sure we are fully aligned and getting the maximum impact from the measures we have implemented," the official said.

It's a critical moment for Biden, Europe and the world. Harsh coordinated sanctions already imposed by the West have not stopped Putin's invasion, which is entering its second month. Biden warned as he was departing the White House for Brussels that chemical warfare posed a "real threat" in Ukraine. And massive refugee flows are quickly turning into a humanitarian crisis for Ukraine's neighbors.

Leaders were expected to address all those issues Thursday, hoping above all to signal to the world their unity and collective commitment to stopping Russia's aggression.

Biden's aides said ahead of the talks that he wanted to strike agreements on new military assistance to Ukraine, new steps to tighten the economic noose on Russia and new measures bolstering NATO's force posture along the alliance's eastern edge as part of his meetings.

"How we make sure that we are continuing to support Ukraine and its effort to defend itself will be a topic of conversation among the leaders," national security adviser Jake Sullivan told reporters aboard Air Force One as Biden was flying to Brussels.

Biden also plans to announce the US will accept up to 100,000 refugees fleeing the war in Ukraine, a senior administration official said, a major step toward easing a brewing humanitarian crisis in Europe. The official said a "full range of legal pathways" would be utilized to welcome the refugees.

Leaders from Poland in particular -- where Biden will visit Friday -- have called on the United States to expedite processing of refugees with family in the United States. More than 3 million people have fled fighting in Ukraine, the UN Refugee Agency says.

'We must be intelligent" with sanctions against Russia, European allies warn

Still, Biden has made clear his options to stop bloodshed in Ukraine are limited. He has drawn the line at sending US troops into direct conflict with Russians and indicated that Ukrainian requests for a NATO-enforced no-fly zone are a nonstarter.

European leaders have also made plain their own limitations in punishing Russia. While the US has imposed a ban on imports of Russian energy products, Europe remains far more dependent and has stopped short of cutting itself off completely.

"We do not have exactly the same situation in Europe and in the United States," Charles Michel, the European Council president, acknowledged in an interview with CNN's Christiane Amanpour on Wednesday. "The oil or the gas sector, for instance. We are much more dependent in Europe in comparison with the situation in the United States."

"It's why we must be intelligent," Michel said. "The goal is to target Russia; the goal is to be painful against Russia. The goal is not to be painful for ourselves."

Leaders who began arriving in Brussels late Wednesday hoped to focus on what they are willing to do rather than what they have ruled out. Biden was expected to announce more American help in weaning Europe from its dependence on Russian natural gas as part of his meetings this week, US officials said.

Sullivan said Biden would reveal details of the assistance on Friday. He added that US and European officials have held an "intense back-and-forth" about reducing dependence on Russian energy in the lead-up to the emergency summits.

"We are aiming at having a commitment for additional supplies for the next two winters," Ursula von der Leyen, the president of the European Commission, told lawmakers Wednesday.

More troops headed to eastern NATO countries

A day ahead of the extraordinary NATO summit, the alliance's secretary general said he expected members to ramp up forces in countries closest to Russia.

"I expect leaders will agree to strengthen NATO's posture in all domains, with major increases of forces in the eastern part of the alliance, on land, in the air and at sea. The first step is the deployment of four new NATO battle groups in Bulgaria, Hungary, Romania and Slovakia," Jens Stoltenberg told reporters ahead of the summit.

Before Biden's departure for Brussels, the Pentagon provided the White House with a series of options for potential additional US troops in Eastern Europe, according to a US official. The US already added troops in Poland and Romania as tensions between Russia and Ukraine escalated.

Sullivan said leaders on Thursday would ratify certain decisions on increasing NATO's troop posture and would task their military and political officials with setting out a "longer-term game plan for what forces and capabilities are going to be required in those eastern flank countries."

That plan will be agreed to at this summer's NATO summit in Madrid, which had been previously announced. The new force posture will ensure that "we've got a long-term footprint that is matched to the new security reality that's been created both by Russia's full-scale invasion of Ukraine and by what's happening in Belarus," Sullivan said.

Officials said possibilities for changes include more forward-deployed US troops, either permanently or on a rotating basis, which could lead to more and potentially larger field exercises; a more structured rotational presence within the NATO force structure; or the construction of a new traditional US military base.

This story has been updated with additional details Thursday.