President Donald Trump said Wednesday Iran must quickly negotiate over its nuclear program or risk U.S. military action, prompting Tehran to warn it would respond “like never before” to any strike.

Trump has not ruled out an attack after this month's deadly crackdown on protests. Last June, the U.S. carried out a night of strikes on Iranian nuclear sites during Israel's 12-day war against Iran.

A U.S. naval strike group that Trump described as an "armada" led by aircraft carrier the USS Abraham Lincoln is now lurking in Middle East waters.

A rights group said that it has verified over 6,200 deaths, mostly of protesters killed by security forces, in the wave of demonstrations that rocked the Iranian government since late December but peaked on Jan. 8 and 9.

Activists say that the actual toll could be many times higher, with an internet shutdown still complicating efforts to confirm information about the scale of the killings.