Russian foreign minister Sergei Lavrov on Tuesday warned Kiev against using force to quell pro-Moscow separatists in eastern Ukraine, saying the "criminal" act would undermine talks planned in Geneva.

The four-way meeting set for Thursday involving top diplomats from Russia, Ukraine, the United States and the European Union is the latest step in a flurry of diplomacy aimed at easing the worst European security crisis in decades.

"One cannot issue invitations to talks while at the same time issuing criminal orders for the use of armed force against the people there," Lavrov said during a visit to Beijing.

"You can't send in tanks and at the same time hold talks, and the use of force would sabotage the opportunity offered by the four-party negotiations in Geneva," he said.

US Assistant Secretary of State Victoria Nuland earlier played down US expectations for the summit, although she maintained that "it is very important to keep that diplomatic door open and will see what they bring".

In unusually strong language at a joint news conference with his Chinese counterpart Wang Yi at Beijing's Diaoyutai State Guesthouse, Lavrov denounced Kiev for "spreading lies" about Russia's position and actions in Ukraine.

Moscow "in principle" supports the idea of four-way talks, although they must be "genuine and not merely for show," Lavrov said.

"So, if the Ukrainian foreign ministry says Russia is afraid to hold these talks in Geneva, don't believe it," he said. "It's a lie."

Lavrov also denied international claims that Russia is dispatching pro-Kremlin forces to Ukraine's southeast, where balaclava-clad gunmen have stormed government buildings.

"This is a total lie that supposes that those residents there are completely incapable of protesting of their own will," he said.

Lavrov's visit, which includes a meeting later Tuesday with President Xi Jinping, comes amid preparations for Russian President Vladimir Putin's trip to China next month, as the two powers forge increasingly close ties.

In a sign of warming relations, Xi made Russia his first destination after taking office last year and attended the Sochi Olympics in March, becoming China's first leader to attend a major overseas sports event.

The Ukraine crisis has thrown up an unanticipated hurdle, however. Beijing has struggled to support its ally while maintaining its stance of "non-interference" in other countries' domestic affairs.

Beijing has refrained from publicly criticising Russia on the issue. And when the UN General Assembly last month adopted a Ukraine-backed resolution condemning Crimea's referendum and refusing to recognise Russia's annexation of the peninsula, China abstained from voting.

Lavrov told reporters at the news conference with Wang that Russia "appreciates China's objective, balanced and responsible stance" on the Ukraine issue.

Echoing comments he made on Monday at a meeting with German Foreign Minister Frank-Walter Steinmeier, Wang said China welcomes this week's four-way talks in Geneva.

"We're optimistic this will advance the cause of peace that we advocate," Wang said.