President Donald Trump has tweeted that Iran or its proxies “are planning a sneak attack on U.S. troops” and has threatened to exact “a very high price,” if it happens.

Trump has not provided more details, except saying that his statement is based upon “information and belief”.

U.S. intelligence about a potential Iran-backed attack against U.S. personnel and facilities in Iraq suggest that it would likely be a deniable attack, as opposed to the kind overt missile strike that Tehran carried out in January, a U.S. official told Reuters on Wednesday.

The official, who spoke on condition of anonymity, said that intelligence threads about the potential attack by Iran or Iran-backed forces had been building for some time. The official did not disclose intelligence on the timing or precise locations of the attack.

Iranian officials have been warning in recent days that the United States is preparing an attack on its proxy forces in Iraq. The foreign ministry warned April 1 that U.S. is making threatening moves in Iraq, instead of leaving the country as Iraqis “demand”.

The Islamic Revolution Guard Corps (IRGC) also issued a warning March 31, making its own threat of grave consequences for the United States.

In the meantime, reports said on Monday the U.S. increased the number of its Patriot anti-air defense systems around Iraqi military bases where American troops are deployed. The U.S. has also relocated its forces from four bases, in a move seen by Iran and its proxies as preparation for some sort of military action.

The New York Times and Washington Post reported in recent days that Pentagon is planning how to deal with Iran’s proxy forces in Iraq, which have been intermittently firing missiles and mortars at American targets since last year.

However, Reuters reported April 1 that there are widening rifts among Iran-backed Iraqi militias following the killing of Qassem Soleimani and a top Iraqi militia commander by a U.S. drone attack on January3.