Top US diplomat Antony Blinken on Thursday warned Russia against invading Ukraine, as Moscow told Kiev that any attempt to retake the Crimean peninsula amounted to a "direct threat" to Russia.

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 Russia's plans for renewed aggression against Ukraine," Blinken told Lavrov in talks on Thursday near Stockholm, warning of "serious consequences" if Russia "decides to pursue confrontation".

Striking a conciliatory note, Blinken said the US was ready to "facilitate" the "full implementation" of the Minsk peace accords.

The Minsk deal was reached after Russia's 2014 annexation of Crimea and aimed at resolving the conflict with pro-Russia separatists in eastern Ukraine but never enforced.

"The best way to avert a crisis is through diplomacy," Blinken said.

Nightmare scenario

The US, Russia and Ukraine were all in Stockholm on Thursday for a meeting of the Organisation for Security and Cooperation in Europe (OSCE), one of the few international dialogue forums to which both the US and Russia belong. 

Speaking in the plenary session, Blinken called on Moscow to "de-escalate, reverse the recent troop build-up (and) return forces to normal peaceful positions."

Lavrov meanwhile warned that the "nightmare scenario of a military confrontation was returning" in Europe, accusing NATO of inching its military infrastructure closer to Russia's borders.

He reiterated his opposition to any NATO expansion eastward -- including Ukraine -- but said Russia was open to dialogue.

"We are interested in joint efforts toward a resolution of the Ukraine crisis," he said. 

Despite the cordial tone, the OSCE meeting took place amid spiking tensions.

At a NATO meeting in the Latvian capital Riga on Wednesday, Blinken accused Russia of trying to "destabilise 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, including "a range of high-impact economic measures that we've refrained from using in the past".

Blinken also held talks on Thursday with Ukrainian Foreign Minister Dmytro Kuleba, who said his country would "demonstrate restraint".

But he called on Kiev's partners to "prepare a deterrence package" that would make Russia think twice before resorting to military force.

Ukrainian President Volodymyr Zelensky had said on Wednesday that Crimea was Ukrainian territory and Kiev's goal was to "liberate" it.

"We see this as a direct threat to Russia," Kremlin spokesman Dmitry Peskov told reporters on Thursday.

Outside the diplomatic forum, tensions continued on the ground 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 was planned long in advance but comes at a crucial moment as tensions mount on numerous issues in Europe.

In addition to Ukraine, other issues on the meeting's agenda 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.