Russia has said it will respond if its "legitimate interests" are attacked, as Ukraine announced it had ended an Easter truce and was relaunching "anti-terrorist" operations against pro-Russian separatists on its territory.
In an interview with Russia Today due to be aired on Wednesday, Sergey Lavrov, the Russian foreign minister, said that "Russian citizens being attacked is an attack against the Russian Federation".
He referenced the Georgian war of 2008, where Russia invaded Georgia after Tblisi attacked rebels in the breakaway Georgian region of South Ossetia, which had aligned itself with Moscow.
"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 accordance with international law."
The comments were released after the Vitaly Yarema, Ukraine's acting first deputy prime minister, said "appropriate measures" were being taken against pro-Russian separatists in the country's east.
"We have obtained the support of the United States, that they will not leave us alone with an aggressor. We hope that in the event of Russian aggression, this help will be more substantive."
The announcement came a day after the US vice president, Joe Biden, ended his two-day visit to Ukraine, the highest profile US politician to have travelled to the country.
Lavrov said that it was no coincidence that the resumption of operations came after Biden's visit, and charged that the US was "running the show".
The latest warning came after Ukraine's acting president, Oleksandr Turchynov, on Tuesday said: "I demand the resumption of effective counter-terrorism measures to protect Ukrainian citizens in the east from terrorism."
Turchynov said that bodies with signs of torture had been found near the city of Slovyansk, which is in the hands of pro-Russian separatists.
One was that of Volodymyr Rybak, a member of Turchynov's Batkivshchyna party, who had recently been abducted by "terrorists", the president said.
"These crimes are being carried out with the full support and indulgence of the Russian Federation."
A video released on Wednesday showed Rybak confronted by an pro-Russian crowd as he tried to remove a "Donetsk Republic" banner from a building in Horlivka. Ukraine authorities say he was kidnapped hours later, tortured and murdered.
The US also announced on Tuesday that it was sending 600 troops to Poland to begin a series of military exercises in four countries across eastern Europe.
The exercises are part of an effort announced last week aimed at reassuring NATO allies of the US's commitment to the region's defence.