Slovakia initially agreed to supply Ukraine with a key Soviet-era air defense system to help it defend against Russian airstrikes, according to three sources familiar with the matter, but the United States and the NATO is still wondering how to strengthen the defensive capabilities of this country and the transfer is not yet assured.

According to two of the sources, Slovakia, one of the three NATO allies that owns the defense systems in question, wants assurances that the systems will be replaced immediately.

If a country supplied its S-300s, the supplier country would likely receive the US-made Patriot air defense missile system to replace the capability it would give up, according to two other sources familiar with the negotiations.

Germany and the Netherlands have already publicly announced that they will send Patriots to Slovakia. But integrating a complex new air defense system into a country’s existing military architecture, as well as training its forces in its use, can take time, a source familiar with the matter warned.

The push to get more S-300s into Ukrainian hands comes as Congress pressured the Biden administration to help Ukraine get the air defense system. Lawmakers from both parties, who heard Ukrainian President Volodymyr Zelensky in a speech Wednesday morning, urged the United States to do more to help Ukraine get the weapons it seeks, especially after the administration opposed a plan last week to supply Ukraine with Polish MiGs. -29 jets.

Representative Michael McCaul, the top Republican on the House Foreign Affairs Committee, hinted publicly on Wednesday that the United States has made progress in providing Ukraine with access to additional S-300s, a system of anti-aircraft weapons which, according to congressional sources, Ukraine is already operating effectively. against the onslaught of Russia.

“I pushed hard for this,” McCaul told CNN’s Jim Sciutto. “I’m proud to say they have S-300s in the works.”

A McCaul aide later said he was referring to S-300 systems owned and operated by Ukraine for years. These systems are already in the country.

More background: CNN previously reported that the State Department is working to identify countries that currently have S-300s and how they could be transferred to Ukraine.

CNN reported earlier Wednesday that other Soviet-era air defense systems, including the SA8, have already been sent to Ukraine.

“People are talking about a no-fly zone, they can create their own if we give them the military equipment and weapons,” McCaul noted.

Defense Secretary Lloyd Austin is due to travel to Slovakia later this week after attending the NATO Defense Ministerial meeting in Brussels.

“At the request of President Zelensky, we have identified and are helping Ukraine acquire additional longer-range anti-aircraft systems and the ammunition for those systems,” US President Joe Biden said in remarks detailing Wednesday. the new safety aid.

Some U.S. allies have also been extremely reluctant to go public with their contributions to Ukraine, multiple sources told CNN. Bulgaria and Greece also have the more modern S-300 systems in question. The Greek system is a different model from those currently applied by Ukraine, which raises the question of whether additional training would be required for it to be useful.

The State Department and the Slovak Embassy in Washington declined to comment. CNN has reached out to the National Security Council and the Department of Defense for comment.