President Donald Trump has told lawmakers it is not yet possible to know the "full scope and duration" of US strikes on Iran.

In a letter notifying Congress of the military action, Trump said the threat from Iran had been "untenable" despite efforts to find a diplomatic solution.

But lawmakers said they were unclear about Trump's plans after receiving a classified briefing from top officials about the strikes, on the eve of a vote on a war powers resolution that could limit the president's actions.

The US and Israel began striking Iran on Saturday. Iran responded by firing missiles and drones at Israel and Gulf states allied to the US.

In the letter, Trump told Senate President Pro Tempore Chuck Grassley that Iran "remains one of the largest, if not the largest, state-sponsors of terrorism in the world".

He said it "continues to seek the means to possess and employ nuclear weapons".

"Its array of ballistic, cruise, anti-ship and other missiles pose a direct threat to and are attacking US forces, commercial vessels and civilians, as well as those of our allies and partners," the letter added.

The president also said no US ground forces had been committed to the operation and that it was designed to minimise civilian casualties.

Democratic lawmakers - and a handful of Republicans - have questioned whether Trump violated the law by launching strikes without congressional approval. Many have supported efforts to limit Trump's ability to take further action in Iran.

Members of Congress attended a classified briefing on Tuesday from Secretary of State Marco Rubio, Defence Secretary Pete Hegseth and others.

While the president has broad authority to launch military action without a formal declaration of war, Congress, by law, must be notified within 48 hours of hostilities beginning.

Trump has previously conducted military operations without congressional approval, such as the US strikes against Iranian nuclear facilities last year, and the operation to seize Venezuela's President Nicolás Maduro in January.

House of Representatives Speaker Mike Johnson, a Republican, said the Trump administration had notified the Gang of 8 - a bipartisan group of congressional leaders - ahead of the strikes in Iran and that Rubio had briefed those congressional leaders within the required timeframe.

"We've complied with the law 100% and we're going to continue to comply with it," America's top diplomat said.

The secretary of state has also said "there's no law that requires the president to have done anything with regards to this".

"To begin with, no presidential administration has ever accepted the War Powers Act as constitutional – not Republican presidents, not Democratic presidents," he added.

But some lawmakers were unconvinced.

"This is as serious as it gets," Senator Chris Murphy, a Democrat from Connecticut, said after Tuesday's briefing.

"They told us in that room that there were going to be more Americans that were going to die.

"We have to have a debate in the United States senate on the authorisation of military force."

Other lawmakers said afterwards they felt in the dark as to how long the conflict might last, what the objectives were, and whether the US military could become bogged down.

Senator Richard Blumenthal, a Connecticut Democrat, said: "I am more fearful than ever after this briefing that we may be putting boots on the ground.

"And that troops from the United States may be necessary to accomplish objectives that the administration seems to have, but I also am no more clear on what the priorities are going to be of the administration going forward."

Senator Brian Schatz, a Democrat from Hawaii, said he was also confused.

"There are times when you go into a classified session and you walk out with a better understanding about the rationale behind the military action," he said.

"This is not one of those times. We remain as confused as the American people are."

Some lawmakers indicated they might not support the war powers resolution for now, but might change course if the war continues past the four or five weeks, a duration Trump has mentioned.

The vote in the Senate, expected on Wednesday, could be close as a handful of Republicans said they were not sure how they would vote. A majority of Democrats are expected to back the measure.

In the House, there is more bipartisan support for the war powers measure and Republicans have an even narrower majority. Their vote is expected on Thursday.

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