The United States said on Sunday that combined US-Israeli military operations in Iran could continue for a month, despite concerns about the legality of the attack.

"As strong as it (Iran) is, it's a big country, it'll take four weeks — or less," President Donald Trump told the Daily Mail.

However, the legitimacy of the operation has come into question.

Republican Representative Thomas Massie of Kentucky expressed his opposition to the war and criticized the administration's decision. "This is not 'America First',"Massie posted on X, calling it "acts of war unauthorized by Congress".

Former Republican representative Marjorie Taylor Greene, once a staunch Trump supporter, also denounced the strike.

"Thousands and thousands of Americans from my generation have been killed and injured in never ending pointless foreign wars and we said no more," she posted to social media. "It's always a lie and it's always America Last."

Republican Representative Pat Harrigan of North Carolina claimed the 1973 War Powers Resolution gives the president the authority to act for up to 60 days without the need to consult Congress, framing the strikes on Iran as legally defensible.

However, Democrats heavily criticized the move.

"If you're going to initiate war, you need Congress," Senator Tim Kaine told Fox News.

"The president not only did not come to Congress to seek a debate or vote, he acted without even notification to the vast majority of us," Kaine said, calling it "an illegal war".

Secretary of State Marco Rubio and top defense officials are scheduled to brief Congress on the situation on Iran on Tuesday, CBS News reported.

The administration spoke about the ongoing operation on Sunday afternoon and acknowledged it is "likely" that more US service members will die.

On Monday, the US Central Command said four US service members have been killed during Iran's initial attacks and four others have been injured.

Analysts believe the death of Iran's supreme leader Ali Khamenei will likely lead to significant shifts in the regional geopolitical landscape and prolonged chaos.

"Since the United States and Israel violated one of Iran's red lines by assassinating the supreme leader, this gives Iran a reason to escalate the conflict and broaden the scope of its targets," said Abu-Bakr Al-Desouky, an Egyptian expert on Gulf affairs and Iranian politics.

Adnan Bourji, director of the Lebanese National Center for Studies, agreed that "the war is still in its early stages, and so far there are no clear indications that it will end very soon".

Marc Weller, director of the Global Governance and Security Centre at Chatham House, said that by attacking Iran, Washington has taken a further, major step in upsetting the global order.

"The core principle of that order is that no state can go to war in pursuit of its own national policy. Where use of force is claimed as necessary in the global interest, this can only be done through a mandate from the (United Nations) Security Council," Weller wrote in an article on Chatham House's website on Sunday.

Shooting in Texas

Meanwhile, a gunman in Texas wearing clothes with an Iranian flag design and the words "Property of Allah" killed two people and wounded 14 on Sunday, a law enforcement official told The Associated Press.

Police in Austin said they shot and killed the gunman, who used both a pistol and a rifle to carry out the attack. He was identified as 53-year-old Ndiaga Diagne, the Department of Homeland Security said in a statement.

FBI special agent Alex Doran said an exact motive was not known but "there were indicators on the subject and in his vehicle that indicate a potential nexus to terrorism".

Xinhua and agencies contributed to this story.

Contact the writers at bilinlin@chinadailyusa.com