Originally Published: 16 MAR 23 10:16 ET

Updated: 16 MAR 23 12:13 ET

By Antonia Mortensen, Duarte Mendonca and Luke McGee, CNN

(CNN) -- Poland on Thursday pledged it would send four MiG-29 fighter jets to Ukraine, the first NATO member to do so, in a significant move in Kyiv's battle to resist Russia's onslaught.

"When it comes to the MiG-29 aircraft, which are still operating in the defense of Polish airspace, a decision has been taken at the highest levels, we can say confidently that we are sending MiGs to Ukraine," President Andrzej Duda said.

Poland's decision to send the jets to Ukraine marks a break from the NATO alliance, and may put pressure on other member states to do the same. Other NATO nations have been reluctant to move far beyond a decision earlier this year to send tanks to Kyiv.

Duda said the planes were among about a dozen that it had inherited from the former German Democratic Republic, and were coming to the end of their operation life but were "still functional." They would be handed over in the coming days after being serviced.

Speaking at a news conference in Warsaw with his new Czech counterpart Petr Pavel, the Polish president expressed the two countries' joint backing for Kyiv.

"The Czech Republic and Poland are countries that are in the absolute vanguard when it comes to supporting Ukraine, both at humanitarian and military levels," President Duda said.

Poland had been one of the most vocal European nations against Russia -- even before the invasion of Ukraine. Russia is still seen by many in Poland's political and diplomatic circles in a Cold War context. Putin has always been seen by Warsaw as untrustworthy and Russia expansion is something to be fought against at all costs. It is one of the few NATO countries that by law is required to meet its 2% of GDP defense spending commitment and is an active member of the European defense community.

While sending MiGs is a break from the alliance, it is not an unexpected move and complies entirely with Poland's membership of NATO. It might change the dynamic within the alliance, acting as a catalyst for more countries to do so, or upset countries that are opposed to NATO getting more involved in the conflict like Hungary.

The biggest question will be if it puts pressure on the United Kingdom and the United States, which will then do the same for Germany. Ultimately, creating this pressure on other allies was probably Poland's intention.

German Chancellor Olaf Scholz announced his country would provide 14 Leopard 2 tanks in January, bowing to intensifying international pressure, led by the United States, Poland and a bloc of other European nations, which called on Berlin to step up its military support and commit to sending their sought-after vehicles.

The announcement was matched by the US, with President Joe Biden saying that he would provide 31 M1 Abrams tanks to Ukraine, reversing the administration's longstanding resistance to requests from Kyiv for the highly sophisticated but maintenance-heavy vehicles.

In addition to tanks, Ukraine has also been pushing for the US to provide fighter jets, arguing that it needs the planes to defend against Russian missile and drone attacks.

But that push has been met with skepticism by US and allied officials, who say the jets would be impractical because they require considerable training and Russia has extensive anti-aircraft systems that could easily shoot them down.

US and European officials have previously told CNN that F-16 fighter jets were impractical in this situation. Germany ruled out fighter jet deliveries to Ukraine completely while UK government officials echoed the sentiment and said that they believed it was not practical to send jets into Ukraine.

Alleged spy ring

Meanwhile Thursday, Polish authorities said nine people belonging to an alleged espionage ring had been detained, suspected of "collaborating" with the Russian secret service agency FSB.

Interior Minister Mariusz Kamiński said those detained were "foreigners from across the eastern border."

"The suspects conducted intelligence activities against Poland and prepared acts of sabotage at the request of Russian intelligence," the minister said.

Kamiński revealed that the prosecutor's office charged six people with espionage and participation in an organized criminal group.

The court decided to pre-trial detention of the six people, he said, adding that prosecution proceedings are pending against the three detained on Wednesday.

"Evidence shows that the group monitored railway routes. Its tasks included recognizing, monitoring and documenting transports with weapons delivered to Ukraine," the minister said.

"The suspects were also supposed to be preparing for sabotage activities aimed at paralyzing the supply of equipment, weapons and aid to Ukraine," Kamiński continued.