WASHINGTON — Multiple US Democratic lawmakers slammed the administration on Saturday for launching massive military strikes against Iran without Congress' war power authorization, saying the operation raises "serious legal and constitutional concerns".

"By the president's own words,'American heroes may be lost,'" said Senator Mark Warner, vice-chairman of the Senate Select Committee on Intelligence.

"That alone should have demanded the highest level of scrutiny, deliberation and accountability, yet the president moved forward without seeking congressional authorization," Warner said.

"The Constitution is clear: The decision to take this nation to war rests with Congress, and launching large-scale military operations — particularly in the absence of an imminent threat to the United States — raises serious legal and constitutional concerns."

Veteran Senator Tim Kaine called the strikes "a colossal mistake", urging his colleagues to "immediately return" to the Capitol and vote on whether to authorize or limit US strikes against Iran.

"For months, I have raised hell about the fact that the American people want lower prices, not more war — especially wars that aren't authorized by Congress, as required by the Constitution, and don't have a clear objective," Kaine said.

Senator Ruben Gallego said on social media on Saturday that people should not have to "pay the ultimate price for regime change and a war that hasn't been explained or justified to the American people".

Hakeem Jeffries, the top Democrat in the House of Representatives, said the administration "failed to seek Congressional authorization prior to striking Iran".

Any US president, excluding under the "exigent circumstances","must seek authorization for the preemptive use of military force that constitutes an act of war", Jeffries said in a statement.

The ongoing operation "has left American troops vulnerable to Iran's retaliatory actions", he said.