Russian President Vladimir Putin has warned America and its allies against taking one-sided action in Syria.
He said any military strikes without UN approval would be "an aggression".
He says that such an endorsement would require "convincing" evidence that President Bashar al-Assad\’s government used chemical weapons against citizens.
He also says the currently available evidence does not fulfil this criteria.
In a wide-ranging interview with the Associated Press news agency and Russia\’s state Channel 1 television, Putin said it would be "absolutely absurd" for Assad\’s forces to have used chemical weapons at a time when they were in the ascendency in the conflict.
"From our viewpoint, it seems absolutely absurd that the armed forces, the regular armed forces, which are on the offensive today and in some areas have encircled the so-called rebels and are finishing them off, that in these conditions they would start using forbidden chemical weapons while realising quite well that it could serve as a pretext for applying sanctions against them, including the use of force," Putin said in the interview, released on Wednesday.
"If there is evidence that chemical weapons were used, and by the regular army… then this evidence must be presented to the UN Security Council. And it must be convincing," Putin said.
He confirmed that Russia had delivered some components of S-300 missile systems to Syria but deliveries had now been "suspended".
U.S. President Barack Obama won his first major victories on securing Congress support for strikes on Syria, with senior members on both sides of the House of Representatives saying they would back him.
U.S. House Speaker John Boehner and House Minority Leader Nancy Pelosi made the comments after a White House meeting with Obama on Tuesday.
Boehner, leader of the Republican party in the House of Representatives, says the United States has the capability to stop Syrian President Bashar al-Assad and must send a warning to other countries that the use of chemical weapons will not be tolerated.
Nancy Pelosi, who leads Obama\’s Democratic Party in the House, said Syria\’s use of chemical weapons was outside of the "circle of civilized behavior." She said the United States must respond.
The Syrian government has denied chemical weapons use by the military.
Senate Intelligence Committee chairwoman Dianne Feinstein, a Democrat, said she is fully supportive of Obama\’s intended action.
Congressman Eliot Engel, the most senior Democrat on the House Foreign Affairs Committee, said he hopes Obama receives bipartisan support for military action against what Engel called a war crime.
"To turn weapons of mass destruction on your own population is the most despicable thing that anyone can do. If we did not respond, in kind, it would send a message to every despot, every thug, every dictator, every terrorist group in the world that you can commit war crimes and murder your own citizens with impunity and nothing is going to happen. "
Heading into the session, Obama said he was confident he could work with Congress to get a resolution authorizing military action.
He said he was asking Congress to approve a "proportional, limited" military response that would send a "clear message" to Assad\’s regime and any other country interested in "testing international norms."
"We recognize that there are certain weapons that when used can not only end up resulting in grotesque deaths but also can end up being transmitted to non-state actors, can pose a risk to allies and friends of ours, like Israel, like Jordan, like Turkey."
He also said the U.S. had a "broader strategy" that involves upgrading the capabilities of the Syrian opposition.
Later Tuesday, the Senate Foreign Relations Committee convened to hear testimony from Secretary of State John Kerry, Defense Secretary Chuck Hagel and the chairman of the Joint Chiefs of Staff, General Martin Dempsey.
Kerry told the panel that US allies such as Israel and Jordan were "one stiff breeze" away from potentially being hurt by any fresh chemical weapons attacks, and that US inaction would only embolden the Syrian president.
"This is not the time for armchair isolationism," Kerry said. "This is not the time to be spectators to slaughter. Neither our country nor out conscience can afford the cost of silence.
"We have spoken up against unspeakable horror many times in the past. Now we must stand up and act."
There will also be a classified briefing for all members of Congress.
Obama will head to Sweden late on Tuesday for a G20 meeting sure to be dominated by Syria.
In another development, French President Francois Hollande said he will wait to see if Obama receives support from Congress before deciding on a French role in any military intervention. France had earlier voiced support for military strikes against the Syrian government.
Later on Tuesday, the U.N. secretary-general Ban Ki-moon said that military action was only justified if in self-defence or with a UN mandate.
"The use of force is lawful only when in exercise of self-defence in accordance with Article 51 of the United Nations Charter and, or when the Security Council approves such action."
But he said the U.N. Security Council should unite if it was proved that chemical weapons were used in Syria by any side.
"If confirmed, any use of chemical weapons by anyone under any circumstances will be a serious violation of international law and outrageous war crime," he said in a video statement. "Any perpetrators must be brought to justice. There should be no impunity."
He told reporters on Tuesday that a U.N. inspection team that collected samples, last week, at the site of the suspected chemical attack near Damascus is working around the clock to prepare its information.
The number of refugees fleeing Syria\’s bloody civil war has surged past the 2 million mark, with almost 5,000 people crossing into that country\’s neighbors every day, according to a new report from the United Nations Refugee Agency issued Tuesday.
In a statement Tuesday, UNHCR chief Antonio Guterres called the situation "a disgraceful humanitarian calamity" not seen in recent history.
Most of the refugees have gone to neighboring countries, including Jordan, Lebanon, Turkey and Iraq, where governments have had to cope with the strain of hundreds of thousands of people in need.
The U.N. says many of the refugees are leaving Syria with little more than the clothes on their back, and that half of them are children. It has made appeals for international aid, but said Tuesday it has received only 47 percent of the money required to meet the basic needs of the refugees.
With another 4.25 million people displaced within Syria, the conflict has forced more than a quarter of the country\’s population to leave their homes.
On Tuesday, Sweden announced it would become the first European country to grant asylum to all Syrian refugees who apply. They will get permanent resident status.
Sweden has taken in 14,700 asylum seekers from Syria since 2012.
The UN says this is the worst refugee crisis for 20 years, with numbers not seen since the 1994 genocide in Rwanda.
More than 100,000 people are estimated to have died since the conflict erupted in Syria in March 2011, and the conflict has produced at least 1.7 million refugees.