According to international media, the defence ministry statement, which threatened retaliation, came a day after Ukraine said it had hit targets in the region.

Meanwhile Ukraine s air force said Russia launched a record 188 drones in a single attack on Monday night, damaging critical infrastructure.

Tensions have been high since the US reportedly allowed Ukraine to use Atacms missiles on targets inside Russia last week, in response to Russia deploying North Korean troops, as reported by Reuters.

The week culminated with Russia s use of the powerful new intermediate-range Oreshnik ballistic missile on the Ukrainian city of Dnipro.

The first reported strikes by Atacms on Russian territory were reported on Tuesday, when Russia said falling fragments caused a fire at a military facility.

But Monday s strike on an S-400 air-defence missile battalion at Lotarevka northwest of Kursk on Saturday could be seen as more serious. The S-400 is considered the closest Russian equivalent of the US Patriot missile system.

Three of the five Atacms missiles were shot down but two reached the target, damaging a radar system and causing casualties, the Defence Ministry said.

A second strike on Monday on the Khalino (Kursk East) air base caused "insignificant damage" after one of the eight missiles fired by Ukraine got through air defences, it maintained.

The statement said, "The Ministry of Defence of the Russian Federation is in control of the situation and retaliatory measures are being prepared.”

It shared photos of what it said was debris from the air base attack.

Russian military bloggers reported the Khalino attack on Monday.

Also footage posted to social media on Monday - which documented bright flashes in the sky above the border region - claimed to show the moment Atacms missiles were intercepted by Russian air defences elsewhere in the Kursk region.

Russia warned the United States on Wednesday to halt what it called a "spiral of escalation" over Ukraine, but said it would keep informing Washington about test missile launches in order to avoid "dangerous mistakes".

The comments from Deputy Foreign Minister Sergei Ryabkov sent a signal that Moscow, which last week approved a new policy that lowered its threshold for the use of nuclear weapons, wants to keep communication channels open at a time of acute tensions with the U.S.

Ryabkov was speaking six days after Russia launched what it described as a new intermediate-range hypersonic ballistic missile called the Oreshnik against Ukraine - something he said had sent a clear message to the West.

"The signal is very clear and obvious - stop, you should not do this any more, you mustn t supply Kyiv with everything they want, don t encourage them towards new military adventures, they are too dangerous," state media quoted Ryabkov as saying.

"The current (U.S.) administration must stop this spiral of escalation," Ryabkov added. "They simply must, otherwise the situation will become too dangerous for everyone, including the United States itself."

President Vladimir Putin said last week that Russia fired the Oreshnik in response to Ukraine s first use of U.S. ATACMS ballistic missiles and British Storm Shadow cruise missiles to strike at Russian territory with permission from the West.

Ukrainian President Volodymyr Zelenskiy said Russia s use of the new missile - which Kyiv said reached a speed of 13,600 kph (8,450 miles per hour) - amounted to "a clear and severe escalation" in the war and called for strong worldwide condemnation.

The U.S. military said the missile was experimental and that Russia likely possessed only a handful of them.