Iran’s Revolutionary Guards said on Friday Tehran will not negotiate with the United States and denied any US attack was likely, a day after US President Donald Trump urged talks and said he could not rule out a military confrontation.

Trump on Thursday urged Iran’s leadership to sit down and talk with him about giving up Tehran’s nuclear program and said he could not rule out a military confrontation given the heightened tensions between the two countries.

American B-52 bombers sent to the Middle East over what Washington describes as threats from Iran have already arrived at a US base in Qatar, said US Central Command (CENTCOM).

“No talks will be held with the Americans and the Americans will not dare take military action against us,” Yadollah Javani, the Guards’ deputy head for political affairs, was quoted as saying by Tasnim News Agency. “Our nation ... sees America as unreliable.”

Thousands of Iranians took part in marches on Friday to voice support for the government’s move on Wednesday to scale back curbs to its nuclear program under a 2015 deal with world powers, and threatened to do more if signatories did not shield it from US sanctions.

State television showed thousands marching after Friday prayers in Tehran and said similar marches were held across the country.

“America should know, sanctions have no effect!” chanted the demonstrators, many of whom carried signs reading “Down with USA”.

Trump, who last year pulled Washington out of the Iran nuclear deal and reimposed sanctions on Tehran, has expressed a willingness to meet Iranian leaders in the past to no avail and renewed that appeal in talking to reporters on Thursday.

Asked about Trump’s comments, Iran’s ambassador to the United Nations Majid Takht Ravanchi said Iran had been talking with the six powers, including the United States, within the framework of the nuclear deal.

“All of a sudden he decided to leave the negotiating table. [...] What is the guarantee that he will not renege again?” Takht Ravanchi said in a US television interview.

He dismissed US allegations of an Iranian threat as “fake intelligence” and said they were “being produced by the same people who in the run-up to the US invasion of Iraq did the same”.

Trump was asked by reporters whether there was a risk of military confrontation with the US military presence in the area.

“I guess you could say that always, right? I don’t want to say no, but hopefully that won’t happen. We have one of the most powerful ships in the world that is loaded up and we don’t want to do anything,” he said.

The media officer at Al Udaid air base, near Doha, did not immediately respond to a phone call and email requesting comment and details about the arrival of the B-52 bombers. CENTCOM is responsible for US military operations in the Middle East and Afghanistan.