US Secretary of State John Kerry said tonight that a resolution in Congress on the use of military force in Syria should not remove the option of using US ground troops.

However, he stressed there was "no intention" of inserting American soldiers into Syria's civil war.

At the first public hearing in Congress on potential military action in Syria, Mr Kerry said "it would be preferable" not to preclude the use of ground troops.

He said it was better to preserve President Barack Obama's options if there was a potential threat of chemical weapons falling into the hands of extremists.

"I don't want to take off the table an option that might or might not be available to a president of the United States to secure our country," Mr Kerry told the Senate Foreign Relations Committee.

The circumstances where troops should be used could be "in the event Syria imploded, for instance, or in the event there was a threat of a chemical weapons cache falling into the hands of al-Nusra or someone else, and it was clearly in the interests of our allies and all of us - the British, the French and others - to prevent those weapons of mass destruction falling into the hands of the worst elements," he said.

Al-Nusra is an al-Qaeda-affiliated group that operates in Syria.

Mr Kerry and Secretary of Defense Chuck Hagel were addressing politicians as part of the administration's effort to persuade Congress to back Mr Obama's plan to launch limited strikes on Syria for its alleged use of chemical weapons last month.

Mr Kerry assured politicians that it would be easy to word a resolution on military force to reassure Congress and the public "there's no door open here through which someone can march in ways that the Congress doesn't want it to, while still protecting the national security interests of the country".

Mr Kerry and Mr Hagel told the committee that any military operation would be limited and specifically designed to degrade President Bashar al-Assad's chemical weapons capability.

Mr Hagel added that a failure to punish Syria for the use of chemical weapons would damage US national security interests and American credibility.

"A refusal to act would undermine the credibility of America's other security commitments - including the president's commitment to prevent Iran from acquiring a nuclear weapon," he said.

"The word of the United States must mean something."

As Mr Kerry and Mr Hagel pressed their case for limited military strikes in Syria, Mr Obama won support for action from two top Republicans in the House of Representatives - Speaker John Boehner and Majority Leader Eric Cantor.

"Only the United States has the capability and the capacity to stop Assad and to warn others around the world that this type of behavior is not going to be tolerated," Mr Boehner told reporters.

"I believe that my colleagues should support this call for action."

Significant opposition remains in Congress, where many politicians, including Mr Obama's fellow Democrats, have said they are concerned the president's draft resolution could be too open-ended and allow possible use of ground troops or eventual attacks on other countries.

The resolution authorises Mr Obama to use military force as necessary to "prevent or deter the use or proliferation" to or from Syria of any weapons of mass destruction, including chemical weapons.

Mr Obama said on Saturday he would seek politicians’ approval for a possible military strike, slowing what had appeared to be plans for a swift action.

Polls show strong public opposition to US action.

A Reuters/Ipsos poll released today showed Mr Obama has failed to convince most Americans of the need for a military strike in Syria.

56% of those surveyed said the United States should not intervene in Syria, while only 19% favoured action, the online poll found.

Ban questions legality of planned US action

Meanwhile, UN Secretary-General Ban Ki-moon has said that the use of force is only legal when it is in self-defence or with UN Security Council authorisation.

His remarks appear to question the legality of US plans to strike Syria without UN backing.

Mr Ban said that if UN inspectors confirm the use of chemical weapons in Syria, the Security Council, which has long been deadlocked on the Syrian civil war, should overcome its differences and take action.

"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 told reporters.

"Any perpetrators must be brought to justice. There should be no impunity."