Slovakia has preliminarily agreed to provide Ukraine with a Soviet-era air defense system, CNN reported, citing sources.

But the U.S. and NATO are still trying to decide how to make up for Slovakia's own defensive capabilities.

According to two sources, Slovakia, one of three NATO allies that have these defense systems, wants guarantees that these systems will be replaced immediately.

If the country provides its S-300s, it will likely receive an American-made Patriot surface-to-air missile system to make up for the capability it is giving up.

Germany and the Netherlands have already publicly stated that they are sending the Patriot to Slovakia. But integrating the new comprehensive air defense system into the country's existing military architecture, as well as training its forces to use it, could take time, warned one source familiar with the matter.

Defense Secretary Lloyd Austin is scheduled to visit Slovakia later this week.

"At the request of President Zelenskyy, we have identified and are helping Ukraine acquire additional longer-range anti-aircraft systems and ammunitions for those systems," U.S. President Joe Biden said Wednesday as part of comments detailing new security assistance to Kiev.