President Donald Trump said on Sunday night that he is considering “very strong options” for a U.S. military response to Iran‘s violent crackdown on anti-regime protesters.

“We’re looking at it very seriously. The military is looking at it, and we’re looking at some very strong options. We’ll make a determination,” Trump told reporters aboard Air Force One.

Trump also revealed that Iran has reached out about negotiating a possible nuclear deal after Trump withdrew the United States from the previous one during his first term. But he stressed that the U.S. “may have to act before a meeting,” adding that the regime is “starting” to cross his red line, where he previously vowed U.S. intervention if they killed protesters.

“They’re starting to [cross it]. It looks like [it]. There seem to be some people killed that aren’t supposed to be killed. … They rule through violence,” he said.

The president added that he is receiving “hourly” reports on the situation, which is worsening.

Multiple U.S-based human rights groups have indicated that the death toll from the protests is now over 500 just two weeks after they began in late December. Arrests also number in the tens of thousands.

Iran has previously threatened preemptive strikes on nearby U.S. bases if Trump were to greenlight a military response.

Trump warned on Sunday if that were to happen, the U.S. “will hit them harder than they imagine.”