Russia will respond if its interests in Ukraine are attacked, Foreign Minister Sergei Lavrov has said, drawing a parallel with the 2008 Georgian war.
Speaking to Russian state TV channel RT, Mr Lavrov also accused the US of "running the show" in Ukraine - a claim Washington dismissed as "ludicrous".
Russia's foreign ministry also repeated its call for Kiev to withdraw military units from the east of Ukraine.
Ukraine's government faces an armed revolt there by pro-Russia separatists.
Kiev and the West say Moscow commands gunmen in eastern Ukraine - something Russia denies.
In recent weeks, pro-Russian militants have seized administrative buildings in at least a dozen towns in the Donetsk and Luhansk regions.
They have installed their own officials, in similar tactics to those used to take over the Ukrainian region of Crimea earlier this year.
The US has called on Moscow to ask pro-Russian gunmen to lay down their weapons and leave public buildings. It has also urged Russia to tone down its aggressive rhetoric or face further sanctions.
The Kiev authorities responded to the occupation of official buildings and roadblocks by carrying out an "anti-terror" operation in eastern Ukraine.
The military operation became bogged down and was suspended over the Easter period before resuming on Tuesday.
The internal affairs ministry now says it has "freed" the town of Sviatohirsk, north of rebel-held Sloviansk, of pro-Russian armed groups.
No-one was hurt, it said. The ministry's claim has not yet been independently confirmed.
'Like in Georgia'
In his interview with Russia Today, Mr Lavrov said it was "quite telling" that Kiev had re-launched its "anti-terrorist" operation in the east on Tuesday during a visit by US Vice-President Joe Biden.
"If our interests, our legitimate interests, the interests of Russians have been attacked directly, like they were in South Ossetia for example, I do not see any other way but to respond in full accordance with international law," Mr Lavrov said.
Russian fought a brief war with Georgia in the summer of 2008 after Georgia sent troops into the breakaway region of South Ossetia to regain control from the Russian-backed rebels.
Responding to Mr Lavrov's interview, US state department spokeswoman Jen Psaki said "many" of his claims "are ludicrous and... not based in fact".
Thousands of Russian troops have massed along Ukraine's borders in recent weeks.
Mr Lavrov also reiterated accusations that Kiev had failed to fulfil commitments laid out in the 17 April Geneva accord designed to end the crisis.
The Russian foreign ministry statement accused Kiev and the West of distorting the agreement, which called for illegal armed groups to be disarmed.
The ministry said the West insisted only on the need for pro-Russian groups to put down their weapons.
Unrest began in Ukraine last November over whether the country should look towards Moscow or the West.