Russia threatened to attack the potential supplies of the S-300 air defence systems to Ukraine after Slovakia announced that it is willing to support Kyiv with supplies, a report said on Friday.

Russian Foreign Minister Sergey Lavrov said that Moscow “will not allow” the transfer of the S-300 long-range air defence systems that can help defend Ukraine against airstrikes, according to The Kyiv Independent. Lavrov also said that any arms supply to Kyiv would be considered by Moscow as a “legitimate target”.

Earlier Thursday, Slovakia’s defence ministry said that the country is willing to send key Soviet-era air defense system provided that Western allies give them a “proper replacement” to avoid creating a security gap, The Washington Post reported. 

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The US Patriot system is considered to be the equivalent of Soviet-made S-300. Germany and the Netherlands have already publicly announced that they are sending Patriots to Slovakia – which  shares a 98-kilometre border with Ukraine.

“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.”

However, US Defense Secretay Lloyd Austin – who was in Slovakia on Thursday – declined to say whether Washington might be willing to fill the gap.

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“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.

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

The push to get more S-300s into Ukraine’s possession comes as Congress has been pressing US President Joe Biden to help Kyiv obtain the air defense system.

Biden announced on Wednesday that an additional $800 million in security assistance, including weapons to take down Russian planes and tanks, will be provided to Ukraine.