Russia has steered onto a collision course with the United States, warning it may use its power of veto in the UN Security Council to block any resolution paving the way for war on Iraq.
Foreign Minister, Mr Igor Ivanov also appealed for UN weapons inspectors in Iraq to be allowed to set their own deadline for judging President Saddam Hussein's efforts to disarm.
In a BBC World Service interview ahead of talks with British counterpart Mr Jack Straw in London, Mr Ivanov insisted Moscow would not support any measure furthering the possibility of war in a Security Council vote on Iraq.
"If the situation so demands, Russia will of course use its right of veto as an extreme measure," Mr Ivanov said. "Russia would not support any decision that would directly or indirectly lead to a war with Iraq."
At a later news conference with Mr Straw, he also said Russia would not abstain in a vote because Iraq was too grave an issue to opt out of.
"The Iraq question is precisely that sort of question when permanent members of the Security Council should not abstain," he said.
Washington has signalled it will push for a vote next week. France, Russia and China - who hold veto powers at the UN - say UN arms inspectors in Iraq must be given more time.
London and Washington have reserved the right to attack Iraq without a fresh UN mandate. But Mr Ivanov told the BBC it would be a "serious mistake with serious consequences" to go to war without a second resolution.
He insisted unanimity in the Security Council, rather than talk of veto-wielding, was the best way forward.
Ivanov called for binding time limits to be set by UN inspectors, led by chief weapons inspector Dr Hans Blix and the head of the UN nuclear watchdog Mr Mohammed ElBaradei.
Dr Blix and Mr ElBaradei are due to address the Security Council on Friday.