BRATISLAVA, March 17 (Reuters) - Slovakia's defence minister said on Thursday (Mar 17) that his country was willing to provide Ukraine with S-300 air defences if NATO allies find a substitute, but his visiting US counterpart told reporters he had nothing to announce on that.

Ukraine has appealed to Western nations for air defences to help repel a Russian military onslaught, now in its fourth week.

"We have been in discussion with US, Ukraine and also other allies on the possibility to deploy, send or give the S-300 to Ukrainians and we are willing to do so," Slovakia's Defence Minister Jaroslav Nad told a news conference.

"But willing to do so immediately when we have a proper replacement."

US Defense Secretary Lloyd Austin, who was speaking alongside Nad, declined to say whether the United States might be willing to fill the gap.

"I don't have any announcements for you this afternoon. These are things that we will continue to work with all of our allies on. And certainly this is not just a US issue, it's a NATO issue," Austin said, without elaborating.

US President Joe Biden on Wednesday announced an additional US$800 million in security assistance, including weapons to take down Russian planes and tanks.

But the kinds of air defences deployed in Slovakia are highly sought after by Ukrainian President Volodymyr Zelenskyy.

NATO member Slovakia has one battery of the S-300 air defence system, inherited from the Soviet era after the break-up of Czechoslovakia in 1993.

It expects to get Patriot missile defences as part of a new NATO battle group that has just been agreed to be deployed in the country as part of NATO's reinforcements on the alliance's eastern flank.

But the Slovaks don't see that as sufficient, given it will not be their system and it is not clear how long it will be based in the country.

Slovakia shares a 98km border with Ukraine.

The country also operates a small fleet of MiG-29 fighter jets, also dependent on Russian servicing.

"We were discussing various options for how to fill in this gap if we decided not to use MiG-29s anymore," Nad said, adding that Slovakia was expected to get Lockheed Martin's F-16 fighter jets in 2024.