(Recasts with briefing on Thursday, more comments, Rogers' letter)
By Patricia Zengerle and Mark Hosenball
WASHINGTON, Aug 28 (Reuters) - Senior Obama administration officials will brief congressional leaders on Thursday on the situation in Syria, congressional aides said, amid complaints by lawmakers they have not been properly consulted as the president deliberates about possible military action.
The briefing by senior White House and national security officials will be with leaders of the Senate and House of Representatives, and the chairmen and ranking members of national security committees, Democratic and Republican congressional aides said.
President Barack Obama has a legal obligation to consult with Congress before sending U.S. forces into harm's way.
The briefing comes as U.S. lawmakers have increasingly complained they should have more of a say in any decision to punish Syria militarily in response to last week's chemical weapons attack on thousands of civilians.
British Prime Minister David Cameron, an ally of the United States in the Syria conflict, is also facing intense pressure from lawmakers, and was forced on Wednesday to push back his timetable for any military strike.
In the United States, more than one-fourth of the members of the House signed a letter demanding that Obama consult with them, and offering to return from their summer recess to debate how the United States should act on Syria.
The House and Senate are not due back from their five-week recess until Sept. 9.
Members of the House and Senate intelligence panels have also expressed displeasure they have not been sufficiently consulted in recent days.
Republican Representative Mike Rogers, chairman of the House Intelligence Committee, sent a letter to Obama on Tuesday asking him to consult formally with Congress.
He said he was one of several members of Congress who had had "informal" briefings with administration officials. "These conversations have not entailed discussion of potential plans or actions being contemplated by your administration," Rogers said, according to a copy of the letter obtained by Reuters.
Democrat Dianne Feinstein, who chairs the Senate Intelligence Committee, said in a statement on Wednesday that she had been briefed by the intelligence community on last week's chemical weapons attack.
John Boehner, the Republican speaker of the House, sent his own letter to Obama on Wednesday asking him "to clearly articulate" to the public and Congress his objectives, policy and strategy for Syria.
"I have conferred with the chairmen of the national security committees who have received initial outreach from senior Administration officials, and while the outreach has been appreciated, it is apparent from the questions above that the outreach has, to date, not reached the level of substantive consultation," Boehner wrote.
Obama late on Wednesday made his case to a war-weary American public for what is looking like an all-but-certain use of force in Syria, casting the need for action based on U.S. national security interests. But he added he and allied leaders had not yet made a decision on military strikes.
AUTHORITY TO ACT
Obama has broad legal powers to undertake military action against Syria. Under the 1973 U.S. War Powers Act, the president must notify lawmakers within 48 hours of launching military action. But forces can fight for 60 days before Congress has to approve any action.
The administration on Wednesday had yet to share a U.S. intelligence report that may directly link the Assad government to last week's attack. A senior administration official said a formal assessment was expected this week and a classified version would be shared with Congress.
Thursday's briefing will be a conference call. But aides said participants without access to secure lines at home could travel to government facilities where they can safely receive classified information.
Many lawmakers, even when complaining about a lack of input, have been careful to add they generally support decisive action against Syria President Bashar al-Assad.
Secretary of State John Kerry delivered an impassioned statement on Monday laying out the case for punishing the Assad government after he said evidence showed that the use of chemical weapons in Syria was "undeniable."
A senior U.S. official said on Wednesday the administration was considering military options that include multi-day strikes on Syrian government targets.
Lawmakers' demands for more of a role have grown as more days pass since last Wednesday's massive chemical weapons attack in Syria.
Republican Representative Scott Rigell asked all 435 members of the House to sign a letter to Obama demanding that he consult with Congress and offering to come back early for debate.
A spokeswoman for Rigell said 116 - 98 Republicans and 18 Democrats - had signed as of late Wednesday.
Other lawmakers have been even more forthright with their reluctance to support military action.
Jim Inhofe, the top Republican on the Senate Armed Services Committee, issued a statement on Wednesday raising questions about how the United States would pay for any intervention.
"Today I told the Administration that I cannot support military action in Syria unless the President presents to Congress his broader strategy in the region that addresses our national security interests and the budget to support it," Inhofe said.
Some Democrats say the United States does not have a good recent track history of achieving its foreign policy goals through military might.
"I'm not sure how we look at the conduct of the wars in Iraq and Afghanistan and believe that we're going to be successful at pulling the strings of Syrian politics through military intervention," Democratic Senator Christopher Murphy told Reuters in a telephone interview on Wednesday. (Additional reporting by Tabassum Zakaria, Susan Cornwell, Roberta Rampton and Susan Heavey; Editing by Karey Van Hall and Peter Cooney)