Russia has strongly criticised the visit to Washington by Ukraine's Volodymyr Zelensky, accusing the US of fighting an indirect war against it.
President Joe Biden has pledged $1.85bn (£1.45bn) of military aid for Ukraine - including an advanced missile system to help guard against Russian attacks.
In a defiant address to US lawmakers, Mr Zelensky welcomed the assistance. But Russia's US ambassador said these "provocative actions" would lead to an escalation with severe consequences.
Anatoly Antonov told Russian state media that Moscow was "trying to appeal to common sense at all levels", but talk of delivering the Patriot missile system to Kyiv was "deeply disturbing".
Patriot missiles are capable, effective and expensive. The White House says they will help defend Ukrainians against "Russia's barbaric attacks on Ukraine's critical infrastructure".
Moscow has complained that no calls for peace were heard during President Zelensky's trip to Washington
with spokesman Dmitry Peskov describing this as proof that the US was fighting a proxy war with Russia.