MOSCOW (NYTIMES) - Russian Foreign Minister Sergey Lavrov on Friday (Dec 31) warned that the Kremlin perceives the United States and its allies as stoking the war in eastern Ukraine, a shift in tone from Moscow just hours after another Russian official had said the Kremlin was satisfied with a phone call between the leaders of the two countries.

"The civil war in Ukraine, ongoing for eight years, is far from over," Mr Lavrov said, in remarks carried by the Russian Information Agency. "The country's authorities don't intend to resolve the conflict" through diplomacy, he added.

"Unfortunately, we see the United States and other Nato nations supporting the militaristic intentions of Kiev, provisioning Ukraine with weapons and sending military specialists," Mr Lavrov said.

Amid high-stakes diplomatic talks over what the US has described as a serious Russian military threat to Ukraine, Mr Lavrov's remarks were the latest in a series of conflicting commentary from the Kremlin that has seesawed between ominous and conciliatory, sometimes within the space of a few days.

Earlier in December, Russian President Vladimir Putin said Moscow might resort to "military technical" means, referring to the use of force, if talks failed.

But after US President Joe Biden and Mr Putin spoke for about 50 minutes Thursday, Mr Yuri Ushakov, Mr Putin's foreign policy adviser, declined to say whether a specific threat of military action had come up.

Although the call ended without clarity on the Kremlin's intentions after massing about 100,000 troops on the Ukrainian border, both sides said it had been constructive.

The call was seen as an effort by both sides to shape the diplomatic landscape before talks on the Ukraine crisis that will begin in Geneva on Jan 10 and then move to Brussels and Vienna later in the week, according to Russian and American officials who briefed journalists.

Russia has issued demands for North Atlantic Treaty Organisation and the US to pull back forces in the region and pledge not to admit new Eastern European members to the alliance.

In Thursday's call, according to American officials, Mr Biden made clear that Western countries would impose harsh sanctions if Russia stepped up military activities along the Ukrainian border. Mr Putin warned that imposing new sanctions could lead to a "complete rupture" in relations.

Officials in both countries had assessed Thursday's conversation positively. "In principle, we are satisfied with the contact, the negotiations, because they have an open, substantive, concrete character," Mr Ushakov told journalists in a briefing early Friday in Moscow.

Mr Lavrov's comments later in the day, in contrast, revived a more confrontational tone. Mr Ushakov had also said concerns about US weaponry provided to Ukraine had come up in the call but emphasised the respectful tone between the two leaders.

On Friday, Mr Biden told reporters that the Russian leader had "laid out some of his concerns about Nato and the United States and Europe. We laid out ours".

Mr Biden added, "I'm not going to negotiate here in public, but we made it clear he cannot, I'll emphasise, cannot invade Ukraine."

After Russian troops massed near the Ukrainian border over autumn, officials in Moscow repeatedly characterised the eastern Ukraine conflict as a pressing security concern for Russia, although it has been simmering for eight years now between Ukraine's central government and Russia-backed separatists.

Analysts have viewed these statements with alarm, as Russian justifications for invading Ukraine.

Russian diplomats call the conflict a "civil war," something Ukraine and Western countries reject as Russian soldiers and special forces fomented the uprising in 2014 and continue to fight on the anti-government side, while Moscow arms and finances what Ukrainians refer to as a combined separatist and Russian force.

American officials have declined to discuss the substance of the talks so far, insisting that, unlike the Russians, they would not negotiate in public.

Russia in December published two draft treaties the foreign ministry said it would like the United States and Nato to sign, publicly staking out positions before even talks had commenced.

A former Ukrainian official and a Member of Parliament in Kiev said, speaking on condition of anonymity, they worry the Biden administration, which has been focused on China as a principal foreign policy concern, is overly wary of antagonising Russia.

That was a dynamic evident in Thursday's call. Mr Putin's threatening of a breach in relations in retaliation for Western sanctions may suggest that the Kremlin has ascertained that Washington is more interested than Moscow in a stable bilateral relationship.

Mr Biden has attempted a two-track approach, trying to deter Russia with unusually specific warnings about imposing a series of sanctions that would go far beyond what the West agreed upon in 2014, after the Russian annexation of Crimea, while simultaneously pursuing the diplomatic negotiations.