Davos, Switzerland - President Donald Trump on Thursday said a "massive fleet" was heading towards Iran, after repeatedly threatening to attack the country in response to a violent crackdown on mass protests.

"We have a massive fleet heading in that direction," Trump told reporters as he flew back to Washington from the World Economic Forum in the Swiss Alpine town of Davos.

"And maybe we won't have to use it ... we have a lot of ships going that direction, just in case, we have a big flotilla going in that direction, and we'll see what happens."

"We have a big force going toward Iran. I'd rather not see anything happen, but we're watching them very closely," Trump said of Iran.

The US is reportedly currently strengthening its military presence in the Middle East, including by deploying the aircraft carrier USS Abraham Lincoln and several accompanying ships.

The nuclear-powered aircraft carrier normally transports several thousand soldiers and dozens of fighter jets and is accompanied by navy destroyers. There has so far been no official confirmation of the deployment from the Pentagon.

Demonstrations began in Tehran in late December amid widespread dissatisfaction with the government, swiftly evolving into a nationwide uprising.

Security forces responded violently, with activists saying more than 4,500 demonstrators were confirmed dead and thousands more cases were still under investigation.

Iranian authorities on Wednesday put the official death toll at 3,117 people, including security forces.

Trump had persistently threatened military intervention, purportedly to support the protesters, before backing down when Iran issued assurances that it would not execute people detained. He and top Iranian officials have, however, continued to snipe at each other with bellicose statements.