Washington DC - The House of Representatives on Thursday rejected an effort to curb President Donald Trump's authority to wage war in Iran, another setback in Democrats' campaign to force Congress back into decisions over military action in the Middle East.

The vote, held after Democrats forced the issue onto the floor, came as unease over the six-week conflict continued to spread on Capitol Hill, with lawmakers wary of rising costs, an unclear endgame and the risk of a wider war.

The measure would have required Trump to end US military operations against Tehran unless Congress explicitly approved them, invoking the 1973 War Powers Resolution that limits presidential freedom to conduct prolonged hostilities without lawmakers' consent.

It was largely a party-line vote, although one of Trump's Republicans supported the resolution and another abstained, while a Democrat voted against it.

Democrats argue that Trump launched the conflict jointly with Israel on February 28 without the congressional authorization required by the Constitution, which gives Congress sole power to declare war.

"We are standing at the edge of a cliff, and Congress must act before this president pushes us off. Every day we delay, we inch closer to a conflict with no exit ramp," Gregory Meeks, the top Democrat on the Foreign Affairs Committee, said ahead of the vote.

The defeat came despite Democrats narrowing the margin from a previous failed House vote in March, when a similar resolution lost by seven votes.

But in the end, Republican support remained too thin.

Trump has so far retained broad backing from his party, despite frustration among some lawmakers over the administration's refusal to publicly detail the financial and military burden of the war.

At congressional hearings on Wednesday and Thursday, White House budget director Russ Vought declined to estimate the taxpayer cost, and would not confirm an estimate by the Senate Budget Committee's top Democrat, Jeff Merkley, that the figure was around $50 billion so far.

Democrats insist failed war powers votes are still valuable because they force lawmakers to go on the record.

The House vote came a day after the Senate again rejected its own war powers effort, though Democratic leaders in both chambers have vowed to keep bringing the issue back.

"Americans are seeing hundreds of casualties in a war that no one has given them the respect to explain," said Democratic Whip Katherine Clark.

"But one thing is clear: Americans are being asked to foot the bill. With the lives of their sons and daughters. And with the $2 billion a day that we are spending on this conflict that could be going to help Americans afford to go see a doctor."