Washington DC - President Donald Trump said Friday he was sending a second aircraft carrier group to the Middle East – warning it would be a "bad day for Iran" if it fails to make a deal on its nuclear program.

Trump has upped the military threat against the Islamic Republic in the wake of Tehran's deadly crackdown by security forces on protests last month that rights groups say killed thousands.

"In case we don't make a deal, we'll need it," Trump told journalists at the White House when asked about reports the USS Gerald R. Ford would be moved from the Caribbean to the Middle East.

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"It'll be leaving very soon. We have one out there that just arrived. If we need it, we'll have it ready, a very big force."

Trump said he believed the talks with Iran would be "successful" but added: "If they're not, it's going to be a bad day for Iran, very bad."

The US leader had already sent one aircraft carrier, the USS Abraham Lincoln, to the Middle East, as part of a fleet of 12 US Navy ships in the region.

The four vessels led by the Ford, the world's largest aircraft carrier, have been in the Caribbean, where US forces captured Venezuelan president Nicolás Maduro in January.

They are not expected to return to their home ports until late April or early May, The New York Times said.