BRATISLAVA, Slovakia — The defense minister of Slovakia, Jaroslav Naj’, says his country would be willing to provide S-300 long-range air defense missile systems to Ukraine under certain conditions.

Mr. Naj’ said at a news conference in Bratislava with visiting Secretary of Defense Austin that the matter is still under discussion.

The Soviet-era anti-air defense systems use long-range missiles that are capable of flying hundreds of miles and knocking down cruise missiles as well as warplanes. They could be valuable in thwarting Russian air attacks on Ukraine.

Mr. Naj’ said such a transfer would be possible if his country received a “proper replacement” for its S-300s or if Slovakia received a “capability guaranteed for a certain period of time.”

He stressed that he could not responsibly transfer the S-300s to Ukraine in a manner that left a gap in his country’s defenses. He said Slovakia is open to making an arrangement that preserves its defenses against air threats.

Mr. Austin declined to say whether the Pentagon was in a position to provide Slovakia with a replacement for its S-300s. “These are things that we will continue to work with all of our allies on, and certainly this is not just a U.S. issue, it’s a NATO issue.”