Russian President Vladimir Putin warned that any US military strike on Syria that circumvents the United Nations would undermine the global body and risk unleashing a wave of terror.
Such military action would "result in more innocent victims and escalation, potentially spreading the conflict far beyond Syria's borders," Putin wrote in an op-ed piece appearing in the New York Times.
"No one wants the United Nations to suffer the fate of the League of Nations, which collapsed because it lacked real leverage. This is possible if influential countries bypass the United Nations and take military action without Security Council authorization," he wrote.
The article appeared on the website of the Times at the same time as US Secretary of State John Kerry headed to Geneva for talks with his Russian counterpart Sergei Lavrov on a Russian plan that could see Syria give up its chemical weapons.
The two diplomats were to hold their talks in the Swiss capital on Thursday.
Putin wrote in his commentary that an American military strike could lead to massive loss of life and would foment unrest throughout the already restive Middle East.
"A strike would increase violence and unleash a new wave of terrorism. It could undermine multilateral efforts to resolve the Iranian nuclear problem and the Israeli-Palestinian conflict and further destabilize the Middle East and North Africa. It could throw the entire system of international law and order out of balance."
Putin maintained his steadfast support of Assad, writing that there was "every reason to believe" that it was not the Syrian government but rebel forces used sarin gas in an August 21 attack in the Damascus suburbs.
Putin asserted that the rebels did so to "provoke intervention by their powerful foreign patrons."
He issued his dire warning one day after US President Barack Obama postponed a threat to strike Syria, after Assad's regime welcomed the Russian plan to gather and destroy its chemical arsenal.
In an address from the White House late Tuesday, Obama said he had asked US lawmakers to delay a vote on whether to authorize military action while Washington studies the Russian initiative.
He made his appeal to US lawmakers after a weeks long build up to war in which he sought congressional approval for military strikes against Syria for using chemical weapons on its own people.
Obama made his threats of strikes in response to the August 21 attack, when Syrian forces allegedly killed 1,400 people in rebel-held areas near Damascus using sarin gas, according to US estimates.
But the US leader in his speech late Tuesday gave assurances that there would be no military force for the moment, given the Russian plan.
"This initiative has the potential to remove the threat of chemical weapons without the use of force, particularly because Russia is one of Assad's strongest allies."
The US president added that there would be no military force used until United Nations weapons inspectors had delivered their report into what happened.
US Secretary of State Kerry said he has already discussed Russia's disarmament plan with Lavrov by telephone and, while Washington remains cautious, he said he found the ideas interesting.