Russia has made a rare admission, saying a key air defense system and air base in the Kursk region had been hit by Ukraine with US-supplied ATACMS missiles.
The MoD statement threatening retaliation comes a day after Ukraine said it had struck targets in the region, the BBC reported.
Meanwhile, Ukraine's air force said Russia launched a record 188 drones on Monday night in a raid that damaged critical infrastructure.
Tensions have been high since the US reportedly authorized Ukraine to use ATACMS missiles on Russian territory last week in response to Russia's deployment of North Korean troops.
The highlight of the week was Russia's use of the new powerful medium-range ballistic missile "Oreshnik". along the Ukrainian city of Dnipro.
The first reports of ATACMS strikes on Russian territory were reported on Tuesday, when Russia said falling fragments caused a fire at a military facility.
But Monday's strike on a S-400 anti-aircraft missile battalion in Lotarevka northwest of Kursk on Saturday could be seen as more serious. The S-400 is considered the closest Russian equivalent to the US Patriot missile system.
The Ministry of Defense said three of the five ATACMS missiles were shot down, but two reached the target, damaging a radar system and causing casualties.
Moscow also said Monday's second strike on the Halino (Kursk) air base caused "minor damage" after one of the eight missiles fired by Ukraine went through air defenses.
"The Ministry of Defense of the Russian Federation is monitoring the situation and retaliatory measures are being prepared," the statement added.
The agency shared photos of what it said was debris from the air base attack.
Russian military bloggers reported the attack on Halino on Monday.
Also footage posted on social media on Monday - documenting bright flashes in the sky over the border region - claimed to show the moment ATACMS missiles were intercepted by Russian air defenses elsewhere in the Kursk region.
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The BBC confirmed the footage was real and was filmed in the city of Kursk, but could not establish whether US-supplied missiles were the source of the flashes.