By Kevin Liptak

CNN White House Producer

WASHINGTON (CNN) -- President Donald Trump, formulating US strategy toward some of the world's most volatile regions, is packing his schedule this week with meetings and phone calls designed to elicit input and advice for how to proceed.

Trump has spoken already with the leaders of China, Japan and Germany. On Monday, he ate lunch with a set of foreign ambassadors and dinner with two outspoken foreign policy hawks. Later this week, amid daily briefings from his military brass, Trump also plans to stage an administration briefing on North Korea for all US senators at the White House.

The rapid-pace sessions come ahead of Trump's 100th day in office, as his administration is working hurriedly to rack up achievements on matters foreign and domestic.

Administration officials declined to say whether Trump planned to formally announce new policies toward Afghanistan, North Korea or Syria before Saturday, when he hits his 100-day mark.

But behind the scenes, his advisers are working urgently to crystallize what, until now, have been loose sets of policy directives that have frustrated some career national security officials and foreign allies.

Afghanistan

Trump is on the verge of making a decision on increasing troop levels in Afghanistan following a comprehensive review of US strategy, according to a senior US official, who declined to speculate when precisely Trump would announce his decision.

Trump's Defense Secretary James Mattis made an unannounced visit to Kabul Monday to meet with Afghan government officials and was due to brief Trump later in the day.

The commander of US forces in Afghanistan, Gen. John Nicholson, has said thousands of more troops are necessary to train Afghan soldiers. Mattis, however, declined to specify Monday what his recommendations to Trump would be.

"I owe some degree of confidentiality on where my thinking's at and what I'm going to recommend once I've compiled my notes from this trip," he said.

Over the dinner table on Monday and Tuesday, Trump planned to solicit counsel from other top policymakers: GOP Sens. John McCain and Lindsay Graham, both outspoken on foreign policy; and Bob Corker, the chairman of the Senate Foreign Relations Committee.

McCain and Graham, who dined with Trump Monday, have long advocated for increasing the number of US in troops in Afghanistan, claiming that troop withdrawals during the Obama administration led to a stalemate in the country. The pair wrote in an op-ed last month arguing for "additional US and coalition forces and more flexible authorities."

"Trump has an important opportunity to turn the page, seize the initiative and take the fight to our terrorist enemies," the two lawmakers wrote in The Washington Post. "To do this, the United States must align ends, ways and means in Afghanistan."

North Korea

Corker, meanwhile, said last week that his Tuesday dinner with Trump would focus partly on North Korea, where Trump's longterm strategy remains uncertain.

"We either deal with this over the next two or three years or we've got to realize that we ourselves are held, to a degree, hostage," Corker, a Tennessee Republican, told the Knoxville News Sentinel editorial board last week.

But in a preview of his conversation with the President, Corker warned against rash US action that could result in a full-blown crisis.

"I will say, on the other hand, a miscalculation on our part would be colossal, could create colossal problems," Corker said. "You'd have Russia, China, Japan and South Korea and ourselves all involved in something that could get way out of hand."

Corker will return to the White House Wednesday alongside all other US senators for a briefing on North Korea from Mattis, Secretary of State Rex Tillerson, Director of National Intelligence Dan Coats and Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff.

Such a session at the White House is exceedingly rare; senators are typically briefed en masse on Capitol Hill. The meeting is set to take place in a secure auditorium in the Eisenhower Executive Office Building, which would allow Trump himself to drop into the briefing.

Trump's intentions for North Korea have remained uncertain, partly by design. He has stated publicly that he's loathe to project a clear view of US military options before making a decision to use them. But the lack of clarity has led to fears in Asia of an inadvertent provocation that triggers an outsized response from the North.

China's role

In a late-night conversation Sunday, China's President Xi Jinping urged Trump to show restraint in dealing with Pyongyang to avoid escalating the standoff into a full-blown military crisis. North Korea is still believed to be preparing for its sixth nuclear test.

Trump has made China central to his strategy in dealing with North Korea, but Xi's admonition reflected the tensions blanketing the region. US allies face deep uncertainty amid vague policy pronouncements from the White House.

That unease was fueled last week when it was revealed that administration statements about the US aircraft carrier Carl Vinson heading to the region in a show of deterrence were premature. The vessel was actually sailing in the opposite direction, but the US claims prompted North Korea to vow retribution nonetheless.

On Monday, Trump's plans for dealing with North Korea were no clearer. During a meeting with foreign ambassadors who sit on the United Nations Security Council, Trump answered questions about his goals in Syria and North Korea -- but declined to offer specific terms for US intervention in either country, according to a person familiar with the conversation.

"It was an open dialogue very much of the members wanting to hear from the President what his plans are, what he was going to continue to do on Syria or not continue to do, North Korea, all those issues are certainly at the forefront," said Nikki Haley, the US ambassador to the UN, after the session.

Later in the day, Trump did not appear to heed Xi's plea to avoid escalation. Asked during a reception for conservative media outlets about Kim Jong-Un's military capacity, Trump diminished the North Korean leader.

"I'm not so sure he's so strong like he says he is, I'm not so sure at all," he said, according to an attendee. The White House later confirmed his remark.