Any "punitive" action taken against Syria for the alleged chemical weapons attack could unleash more turmoil and bloodshed in that nation's civil war, UN secretary-general Ban Ki-moon has warned.

Mr Ban also cautioned nations such as the United States and France that may be considering such strikes that they are legal only in self-defence under the UN Charter or if approved by the UN Security Council.

Russia and China have used their veto power in the Security Council multiple times to stop it taking action against Syrian president Bashar Assad's regime.

President Barack Obama has received key support from leaders in Congress for a potential strike.

Mr Obama is confident he will get the support of the US Congress to authorise military intervention.

The president indicated he is open to changes to his request for congressional support for strikes and said he was serious about consulting with Congress, as long as the resolution sends a clear message to Assad and hampers his ability to use chemical weapons.

He said he wants the American people to know: "This is not Iraq, and this is not Afghanistan." He said action in Syria will be limited and proportional.

Congress is holding its first public hearing about plans for military intervention in Syria as Mr Obama tries to convince the country of the need to respond to the chemical weapons attack.

Secretary of state John Kerry, defence secretary Chuck Hagel and joint chiefs chairman General Martin Dempsey were to appear before the Senate foreign relations committee . A classified briefing open to all members of Congress was planned as well.

Mr Obama surprised the world over the weekend when he announced he would seek congressional authorisation for limited military strikes against Assad's regime.

The US says it has proof that the Assad regime is behind attacks that Washington claims killed at least 1,429 people, including more than 400 children.

Now Mr Obama is sending his top national security advisers to the Capitol for briefings. And he is meeting leaders of the House and Senate armed services committees, the foreign relations committees and the intelligence committees.

He has already won conditional support from two of his fiercest foreign policy critics, Republican senators John McCain and Lindsey Graham.

They said they would be more willing to support Mr Obama if the US sought to destroy the Assad government's launching capabilities and committed to give more support to the rebels.

Mr McCain said he is prepared to vote for the authorisation but he would not support a resolution that fails to change the situation on the battlefield, where Assad still has the upper hand.

Robert Menendez, chairman of the Senate foreign relations committee, said he believes the panel will back Mr Obama if the administration explains "the full case" for the use of force and what it sees as the end result. "Not acting has huge consequences," he said.

After a decade of war in Iraq and Afghanistan, polls show most Americans opposed to any new military action overseas.

Some say Mr Obama still has not presented good enough evidence that Assad's forces were responsible for the chemical attack. Others say the president has not explained why intervening is in America's interest.

Those questions come a decade after the Bush administration badly misrepresented the case that Iraq's Saddam Hussein possessed weapons of mass destruction.

The Obama administration argues that the United States must exert global leadership in retaliating for what apparently was the deadliest use of chemical weapons anywhere over the past 25 years.

The administration argues that the sarin gas attack last month violated not only the international standard against using such weapons but also Mr Obama's "red line," set more than a year ago, that such WMD use would not be tolerated.

Syria's conflict has claimed more than 100,000 lives. The fight has evolved from a government crackdown on a largely peaceful protest movement into a full-scale civil war reminiscent of the one that ravaged Iraq over the last decade.

Ethnic massacres have been committed by both sides, which each employ terrorist organisations as allies.