Donald Trump has threatened to "quickly and fully strike back" if Iran attacks any American citizens or targets, and says any American response could be "in a disproportionate manner".

The threat comes as the world waits to see how Iran will respond to the recent killing of Quds Force leader Qassem Soleimani at a Baghdad airport, sparking major anti-US protests across Iran.

On Twitter, Mr Trump suggested that the threat was a "legal notice" to Congress that he might green light such an attack, and suggested that such notice "is not required". Legal scholars have contradicted that notion, though, saying that the War Powers Resolution regulating the launching of wars does not allow Mr Trump to notify Congress through tweet, and noting that the US Constitution does require congressional approval before the country can be taken to war.

"These Media Posts will serve as notification to the United States Congress that should Iran strike any US person or target, the United States will quickly & fully strike back, & perhaps in a disproportionate manner," Mr Trump wrote.

He continued: "Such legal notice is not required, but is given nevertheless!"

The threat comes just days after Soleimani was killed in a targeted air strike at an airport in Baghdad. Following his death, Iran leaders have threatened to avenge his death, but it is not clear when or how that revenge may come.

Mr Trump has been criticised in the days following the attack for not consulting Congress before the attack, with Democrats questioning the wisdom behind such a drastic measure since previous presidents Barack Obama and George W Bush had likewise had the ability to kill the leader but determined not to because of the risk that it would spark unrest in the Middle East.

Shortly after Mr Trump's threat was posted, the president's legal assertions were challenged by legal scholars. That group includes Oona Hathaway, the founder and director of the Centre for Global Legal Challenges at Yale University.

"This tweet threatens to break several laws. First, the President cannot notify Congress under the War Powers Resolution by tweet," she wrote on Twitter in response. "Second, he claims "[s]uch legal notice is not required." That's not true. Any time the president involves the armed forces into 'hostilities,' he must--at a minimum--notify Congress within 48 hours."

She continued: "Third, he is also obligated to 'in every possible instance . . . consult with Congress before introducing United States Armed Forces into hostilities or into situation where imminent involvement in hostilities is clearly indicated by the circumstances . . .' under the" War Powers Resolution. "Fourth, he is promising a 'perhaps ... disproportionate' strike in response--that's another promise of an international law violation. Any action taken in self defence (the apparent justification for the strikes) must be necessary and proportionate to the threat posed."

And she concluded: ​"That any of this has to be said suggests just how insane this situation has become. Where are the White House, [Department of Justice, Department of Defence and] State [Department] lawyers?​"

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