Moscow and Washington traded fierce diplomatic blows over Syria on Thursday with US charges that Russia was pushing its ally into civil war and the Kremlin accusing the White House of being emotional.

The brisk exchange came just as President Vladimir Putin prepared to face a grilling on Friday from the leaders of Germany and France during his first tour abroad since his May 7 inauguration to a controversial third term.

Russia has made clear from the start that Putin would not be swayed by Western and Arab world anger over his refusal to back action against a Middle East regime that Moscow has held patronage over since Soviet times.

"Russia's position is well-known. It is balanced and consistent and completely logical," Interfax quoted Putin's spokesman Dmitry Peskov as saying.

"So it is hardly appropriate to talk about this position changing under someone's pressure... We know it actually could get much worse than it is," she said.

But US Secretary of State Hillary Clinton used some of her most explicit language to date to indicate that Washington's patience with Moscow was running thin and that urgent action at the UN Security Council on Syria was required.

The Russians "are telling me they don't want to see a civil war. I have been telling them their policy is going to help contribute to a civil war," she told a mainly student audience on a visit to Copenhagen.

"We have to bring the Russians on board because the dangers we face are terrible."

The White House on Wednesday also accused Russia of being on "the wrong side of history" and dispatched the US Treasury's financial intelligence chief to Moscow for further talks.

Russia insists that it is not supporting President Bashar al-Assad's regime but respect for international law and the policy of non-intervention in internal conflicts.

That argument has found less currency with foreign powers following the slaying last weekend of 108 civilians -- almost half of them children -- in the Syrian town of Houla that Moscow partially blamed on both sides.

Peskov said Russia's refusal to back further action against the regime after the Houla massacre and other attacks on civilians was based on an approach "completely free of emotions, which are hardly appropriate here."

But Putin is still expected to face tough talk from both German Chancellor Angela Merkel and French President Francois Hollande during a three-nation beginning Thursday in Belarus -- an authoritarian ally he chose as his first foreign destination as head of state.

But Merkel said Thursday, the day before she hosts Putin, that "Russia has cooperated constructively in the UN Security Council."

"There were always times when we said 'we want to go further' but I think that we have to a certain extent common ground when it's a question of ensuring human rights and finally ending these terrible human rights violations," she said.

Hollande in particular has upset Russia by refusing to rule out foreign military intervention to stamp out nearly 15 months of fighting that observers believe has claimed some 13,000 lives.

Russia's standoff with the West has been accompanied by reports of intense behind-the-scenes negotiations focused on ways Moscow could back down without losing its diplomatic prestige.

The New York Times has reported that one option being promoted in some Washington and Moscow circles involves a transition of power similar to that used to end President Ali Abdullah Saleh's strongman rule in Yemen this year.

The transition would reportedly see Assad cede power to his inner circle for an interim period during which political talks with the opposition would be held.

Moscow could theoretically back this option because its historic relations with Syria depend not on Assad but generations of military and trade contacts that it might potentially keep long-term.

Russia has given no direct indication that it was willing to back such an initiative.

But Foreign Minister Sergei Lavrov noted earlier this week that peace envoy Kofi Annan's six-point plan calls for talks "between the Syrian government and the opposition" rather than Assad himself.

"Russia's goal is to avoid the use of foreign force rather than to defend Assad," said Maria Lipman of the Carnegie Moscow Centre.

"Putin cannot stand pressure. His job is to show that he will not be pressured into anything."