WASHINGTON: Congressional Republicans and Democrats are pressing President Barack Obama to explain why the US military should attack Syria and involve Americans in a deadly civil conflict that has roiled the Mideast.

The Republican House speaker, John Boehner (BAY’-nur), wrote to Obama Wednesday seeking answers as the drumbeat of war grew louder.

Exasperated members of the House and Senate say Obama has failed to make a case for US military action against Syria despite the administration’s conclusion that the Syrian government carried out a large-scale chemical weapons attack against civilians last week.

President Barack Obama said a US response would send a “strong signal” to Syrian leader Bashar Assad for his alleged use of chemical weapons.

US intelligence officials are preparing briefings for Congress on evidence aimed at linking last week’s attack to Assad’s government.

US officials also are seeking more intelligence to bolster the White House’s case for a strike. New hurdles are emerging that appear to be slowing the formation of an international coalition to undertake military action.

Russia has blocked efforts to seek a UN force resolution, and Britain’s prime minister says his country is holding off on joining any military efforts until a UN chemical weapons inspection team releases its findings.

The Obama administration is vowing to take action even without the backing of allies or the UN.

Some lawmakers insisted that Obama needs congressional authorization.