By Jeff Mason

WASHINGTON (Reuters) - President Barack Obama on Tuesday urged quick congressional action authorizing the use of military force against Syria and won the support of leaders from both parties in the House of Representatives for limited strikes against President Bashar al-Assad's forces.

Obama told congressional leaders at a White House meeting that the United States has a broad plan to help the rebels defeat Syrian government forces.

After the meeting, the top two Republicans in the House - Speaker John Boehner and Majority Leader Eric Cantor - and top Democrat Nancy Pelosi said they would back military action against Syria.

"Only the United States has the capability and the capacity to stop Assad and to warn others around the world that this type of behaviour is not going to be tolerated," Boehner told reporters. "I believe that my colleagues should support this call for action."

The support came as Obama ramped up his lobbying effort for military action in response to what Washington says was a sarin gas attack by the Syrian government that killed more than 1,400 people, hundreds of them children, near Damascus on August 21.

"What we are envisioning is something limited. It is something proportional. It will degrade Assad's capabilities," Obama told reporters. "At the same time we have a broader strategy that will allow us to upgrade the capabilities of the opposition."

U.S. Secretary of State John Kerry and Secretary of Defence Chuck Hagel will present the administration's case for U.S. military action at a Senate Foreign Relations Committee hearing, which is set to begin at 07.30 p.m. British Time.

Obama said on Saturday he would seek lawmakers' approval for a possible military strike, slowing what had appeared to be plans for a swift action. He has faced stiff resistance from some lawmakers and polls show strong public opposition to U.S. action.

Support from the leaders of both parties could help Obama make his case, but many U.S. lawmakers, including 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 authorizes 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. Some Democrats said the language should be more limiting to ensure it does not authorize the use of ground troops.

Obama told congressional leaders he was willing to address their concerns about the authorization.

"I look forward to listening to the various concerns of the members who are here today. I am confident that those concerns can be addressed," he said.

"I would not be going to Congress if I wasn't serious about consultations and believing that by shaping the authorization to make sure we accomplish the mission, we will be more effective."

Along with Boehner and Pelosi, Republican Senate leader Mitch McConnell and the chairs of congressional committees that deal with national security and the armed services attended the meeting.

Asked whether he was confident Congress would vote in favour of a strike, Obama said: "I am."

The chairman of the Senate Foreign Relations Committee, Democrat Robert Menendez of New Jersey, said he was confident the resolution would pass Congress, but in amended form.

"I believe that we will get there," he said, although he added: "I think our resolution that we will ultimately see will be far more tailored than what the administration sent us."

'NEED TO ACT'

"At the end of the day, (it) will strike the balance between the need to act and act in a way that meets our goals and not create an open-ended, boots-on-the-ground long-term proposition," Menendez told CNBC in an interview.

Menendez, speaking on CBS News, said he wanted Kerry and Hagel to offer "the full case for the use of force" and detail "what that campaign will look like in broad terms, especially as it relates to the end result."

The committee will also hold a closed hearing on Wednesday to discuss the intelligence related to Syria, Menendez told CBS.

It could then begin debate on a Senate version of the bill on Wednesday afternoon, with the aim of sending it to the full Senate for debate next week.

The House and the Senate return from their summer recess on September 9. Both chambers would have to approve the authorization, and it remains unclear whether the Obama administration has the votes.

While he is seeking congressional approval, Obama has said he does not actually require authorization for a strike on Syria, which has blamed the chemical attack on rebel forces.

Republican Senator John McCain, who attended a White House meeting on Syria on Monday, said Obama "would seriously consider" providing weapons to opponents fighting Assad.

"We discussed ... increasing the capabilities and that means providing not only weapons, but the kind of weapons they need, which are anti-armour and anti-air. AK-47s don't do very well against tanks," McCain told CNN's "New Day."

Both McCain and Menendez said that although any action would be aimed at degrading Assad's capabilities to deliver chemical weapons, it could also undermine the Syrian government itself and shift momentum in favour of the rebels.

(Reporting by Mark Felsenthal, Susan Heavey, Vicki Allen; Writing by John Whitesides; Editing by Karey Van Hall, David Brunnstrom and Vicki Allen)