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 By Raf Sanchez, Washington

While the White House has repeated that planned strikes are a response to the use of chemical weapons and not aimed at regime change, Mr Obama said they were intended to help eventually end the nearly three-year conflict. 

"We have a broader strategy that will allow us to upgrade the capabilities of the opposition, allow Syria ultimately to free itself from the kinds of terrible civil war, death and activity that we've been seeing on the ground," Mr Obama said on Tuesday as he met Congressional leaders at the White House. 

The US president's new language is a victory for Senator John McCain, who offered to help Mr Obama avoid a humiliating defeat in Congress in exchange for promises that the US would do more to back the Syrian rebels. 

Mr Obama's meeting with senior Democrats and Republicans comes as part of a full-scale effort by the administration to convince Congress to support military action in Syria. 

John Kerry, the secretary of state, and Chuck Hagel, the defence secretary, will appear before the Senate this afternoon, giving Congress its first chance to formally test the White House's case for war. 

Members of both parties have complained that the resolution authorising the use of force is too broad and gives the White House a "blank cheque" without setting limits on its action. 

Mr Obama signalled that he was open to allowing Democratic senators to rewrite the authorisation to include an explicit ban on use of American ground troops and a possible end date for military action. 

"I would not be going to Congress if I wasn't serious about consultations," Mr Obama said. 

During his brief remarks in the Cabinet room, Mr Obama also tested one of the key talking points the administration is using to make its case: that failing to punish Syria will embolden Iran and endanger Israel. 

Without explicitly invoking Tehran, Mr Obama said that failing to stand up to the Assad regime "sends a message that international norms around issues like nuclear proliferation don't mean much". 

Bashar al-Assad's chemical weapons stockpiles could fall into the hands of terrorists and "pose a risk to allies and friends of ours, like Israel, like Jordan, like Turkey," Mr Obama said. 

He also addressed major concerns in Congress about the US engaging in yet another conflict in the Middle East saying that his plan was "limited" and "proportional". 

"It does not involve boots on the ground. This is not Iraq, and this is not Afghanistan," he said. 

Mr Obama will fly to Sweden on Tuesday night and then on to Russia for the G20 summit, leaving his senior aides to continuing lobbying reluctant members of Congress to support the attack.