America's top diplomat Antony Blinken is expected to warn his Russian counterpart on Thursday against invading Ukraine, as Moscow warns Kiev against any attempt to retake the Crimea peninsula.
Western powers have been sounding the alarm in recent weeks about Russia massing troops along the border with Ukraine, further stoking tensions in an area where a long-running conflict has already left 13,000 dead.
Moscow, which is accused of backing the separatists fighting Kiev, has denied preparing an attack and accuses NATO of raising the temperature.
"We have deep concerns about the aggressive posture that Russia has taken once again towards Ukraine," Blinken told reporters on Thursday after meeting Ukrainian counterpart Dmytro Kuleba in Stockholm.
Kuleba said his country would "demonstrate restraint" but called on its partners to "prepare a deterrence package" that would make Russia think twice before resorting to military force.
The Kremlin said Ukraine's stated goal of retaking Crimea, which Moscow annexed in 2014, amounted to a "direct threat" to Russia.
US, Russia and Ukraine were all in Stockholm on Thursday for a meeting of the Organization for Security and Cooperation in Europe (OSCE), one of the few international dialogue forums to which both the US and Russia belong.
On Wednesday, Blinken said Russia was trying to "destabilize Ukraine from within as well as large scale military operations."
He said it was not known whether Russian President Vladimir Putin had decided to invade, but added: "We do know that he's putting in place the capacity to do so on short order, should he so decide."
Blinken warned there would be "far-reaching and long-lasting consequences" for Moscow if it pushed ahead with any aggression.
Blinken was expected to reiterate that warning directly to Russian Foreign Minister Sergei Lavrov on Thursday.
"The secretary is going to want to make clear that there is a diplomatic offer and that we think that there is a solution, which is full implementation of the Minsk agreements," a US official told reporters.
The Minsk deal, reached after Russia's 2014 annexation of Crimea, is aimed at resolving the conflict with pro-Russia separatists in eastern Ukraine but has never been enforced.
The OSCE is tasked with overseeing the respect of the accords.
On Wednesday, Ukrainian President Volodymyr Zelensky called for direct talks with Moscow over the festering conflict.
On Thursday, a Ukrainian soldier was killed in clashes with pro-Russian separatists, Kiev said, while Moscow announced it had arrested three Ukrainian spies, including one it said had been planning a terrorist attack.
The OSCE meeting "comes at a crucial moment", US ambassador to the body Michael Carpenter said.
In addition to Ukraine, other issues expected to be discussed include the migrant crisis on the Belarus-EU border and the resurgence of clashes between OSCE members Armenia and Azerbaijan.
The EU reached an agreement Wednesday on new sanctions against Belarus, and the US would follow suit soon, the US State Department said.
The OSCE ministers' plenary session is not expected to lead to the adoption of any major agreements, as unanimity is required.
A planned resolution on Ukraine was scrapped because of a lack of consensus.