Amid signs the U.S. is ready to attack Syria, 140 members of the U.S. House of Representatives have signed onto a letter, demanding President Barack Obama get authorization of Congress before going ahead with the military strike.

"We strongly urge you to consult and receive authorization from Congress before ordering the use of U.S. military force in Syria. Your responsibility to do so is prescribed in the Constitution and the War Powers Resolution of 1973," said the letter, which is published on the website of the Capitol Hill newspaper The Hill.

"Engaging our military in Syria when no direct threat to the United States exists and without prior congressional authorization would violate the separation of powers that is clearly delineated in the Constitution," noted the letter, which was sponsored by Scott Rigell, a Republican Representative from the state of Virginia.

The congressmen also made it clear that Obama's decision to authorize the 2011 U.S. military intervention in Libya without authorization of Congress was "unconstitutional."

They added that Congress is ready to reconvene at Obama's request, if he deems that military action in Syria is necessary. U. S. Congress, which is still on summer recess, is to return on Sept. 9.

In a bid to get support of Congress, Obama spoke Thursday to Republican House Speaker John Boehner by telephone to discuss the possible military response to Syria, a day after the top lawmaker urged the president to make his case for striking Syria.

Obama and Boehner discussed, among others, the legal justification for the operation and its objectives. "Only the president can answer these questions, and it is clear that further dialogue and consultation with Congress, as well as communication with the American public, will be needed," Boehner's spokesman Brendan Buck said in a statement.

In the past days, the Obama administration has been preparing for a military strike on Syria, in response to what it has claimed the Syrian government's use of chemical weapons in the eastern suburbs of Damascus, the Syrian capital, on Aug. 21, which allegedly killed 1,300 people.

Even before the United Nations inspection team, which is on the grounds of the Syrian site of the alleged attack, produces a report, the Obama administration has concluded that it was the Syrian government that is responsible for the chemical attack, without providing hard evidence.

In an interview with U.S. media Wednesday, Obama said that he had not made a decision yet on attacking Syria, while stressing that Damascus will face "international consequences" for the use of mass destruction weapons.

White House spokesman Josh Earnest said Thursday that an American intelligence assessment on the Syrian government's culpability could come out as early as Thursday.

Top military, security and intelligence officials of the Obama administration are to brief leaders of U.S. Congress later Thursday on its intelligence assessment of the use of chemical weapons in Syria.

As part of his ongoing consultations with allies, Obama called German Chancellor Angela Merkel Thursday to discuss the Syria situation and possible international response. He also conversed over the phone with leaders of the United Kingdom, France, Australia and Canada, which are major U.S. allies in its previous wars in Iraq and Afghanistan, as well as the NATO bombings in Libya in 2011, in order to coordinate their positions on Syria.

Obama has said that the U.S. military strike on Syria will be limited and aimed not at regime change but at punishing President Bashar al-Assad government for crossing "a red line" by using chemical weapons.

Press reports speculated that the U.S. will launch sea-launched cruise missiles from its warships deployed in the eastern Mediterranean Sea or possibly stealth bombers.

Reacting to international calls for delaying the military strike on Syria, the U.S. State Department reiterated Thursday that Washington will respond in its own timeline to Syria's use of chemical weapons.

"The president will continue contemplating what decision to take in close consultation with our allies," spokeswoman Marie Harf said at a regular press briefing. "I'm not going to go any further than that, except to say that we make our own decisions in our own timeline."

British Prime Minister David Cameron pledged Wednesday to wait until the UN investigators report their findings to the UN Security Council.