Slovakia has preliminarily agreed to provide Ukraine with a key Soviet-era air defense system to help defend against Russian airstrikes, according to three sources familiar with the matter, but the US and NATO are still grappling with how to backfill that country’s own defensive capabilities and the transfer is not yet assured.

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

If a country provided its S-300s, the supplying country is likely to receive the US-made Patriot air defense missile system to backfill the capability it would be giving up, according to two other sources familiar with the negotiations. 

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

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The push to get more S-300s into the hands of the Ukrainians comes as Congress has been pressing the Biden administration to help Ukraine obtain the air defense system. Lawmakers in both parties, who heard from Ukrainian President Volodymyr Zelensky in a speech Wednesday morning, have urged the US needed to do more to help Ukraine obtain the weapons it’s seeking, particularly after the administration opposed a plan last week to provide Ukraine with Polish MiG-29 jets.

Rep. Michael McCaul, the top Republican on the House Foreign Affairs Committee, hinted publicly on Wednesday that the US had made progress in getting Ukraine access to additional S-300s, an anti-aircraft weapon system that congressional sources say Ukraine is already operating effectively against Russia’s assault.

“I’ve been pushing hard for this,” McCaul told CNN’s Jim Sciutto. “I’m proud to say they do have S-300s going in now.”

An aide to McCaul later said he was referring to S-300 systems that have been owned and operated by Ukraine for years. Those systems are already in the country.