The top Republican in Congress on Wednesday demanded an "unambiguous" explanation from President Barack Obama of his objectives in Syria ahead of any US military strike.
Several members of Congress have expressed reservations about a US attack on Syrian President Bashar al-Assad's regime in response to its alleged use of chemical weapons in an attack last week believed to have killed hundreds of civilians.
But House Speaker John Boehner's letter to the president is the sharpest warning yet against any military action before Obama makes his case to the American people and to US lawmakers.
Boehner sought answers to 14 questions -- notably "What is the intended effect of the potential military strikes?" -- and stressed that it will take presidential leadership to gain congressional and public support.
The White House has consulted with key members of Congress including Boehner, Senate Armed Services Committee chairman Carl Levin and others over the possible options for intervention in Syria, but Boehner insisted more was needed.
"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.
"It is essential that you provide a clear, unambiguous explanation of how military action -- which is a means, not a policy -- will secure US objectives and how it fits into your overall policy."
The White House has insisted it has the authority to launch military strikes against Syria without congressional approval, citing the threat to US national security posed by the use of chemical weapons.
But Boehner said the president should "personally make the case to the American people and Congress for how potential military action will secure American national security interests, preserve America's credibility, deter the future use of chemical weapons and, critically, be a part of our broader policy and strategy."
He also said it was "essential you address on what basis any use of force would be legally justified and how the justification comports with the exclusive authority of congressional authorization under Article I of the Constitution."
Boehner also wanted to know whether Obama was prepared to seek funding from Congress for additional military operations beyond the initial plan; if the administration had contingency plans should Assad retaliate; and what the administration would do "should further humanitarian atrocities occur."