WASHINGTON (Reuters) - U.S. President Donald Trump said on Wednesday that Iran or its proxies planned a sneak attack on U.S. targets in Iraq, and warned they would pay a “very heavy price” but gave no details.

FILE PHOTO: U.S. President Donald Trump addresses the daily coronavirus response briefing at the White House in Washington, U.S., March 31, 2020. REUTERS/Tom Brenner

“Upon information and belief, Iran or its proxies are planning a sneak attack on U.S. troops and/or assets in Iraq. If this happens, Iran will pay a very heavy price, indeed!” Trump said in a post on Twitter.

It was not immediately clear what information Trump was referring to in his tweet, which was posted after he was scheduled to have a 12 p.m. ET (1600 GMT) intelligence briefing.

U.S. intelligence about a potential Iran-backed strike in Iraq suggests it would likely be a deniable attack, as opposed to the kind of overt missile strike that Tehran carried out on Jan. 8, a U.S. official told Reuters on condition of anonymity.

The official said intelligence threads about a 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 any attack.

Speaking before Trump’s tweet, a top military aide to Iran’s Supreme Leader Ayatollah Ali Khamenei warned Washington against “provocative actions” in Iraq, Iranian news agencies reported.

U.S.-Iranian relations have been bitter since the Islamic Revolution toppled the U.S.-backed shah of Iran, Mohammed Reza Pahlavi, in 1979 and ushered in an era of theocratic rule.

While there was a detente with the 2015 Iran nuclear deal, relations have deteriorated with Trump’s decision nearly two years ago to abandon that multilateral agreement and reimpose U.S. sanctions that have crippled the Iranian economy.

Worsening tensions, a Jan. 3 U.S. drone strike in Iraq killed Qassem Soleimani, the head of Iran’s elite Quds Force, as well as Abu Mahdi al-Muhandis, who founded Iraq’s Shi’ite Kataib Hezbollah militia after the 2003 U.S.-led invasion of Iraq.

Iran retaliated with a rocket attack on Iraq’s Ain al-Asad base where U.S. forces were stationed on Jan. 8. No U.S. troops were killed or faced immediate bodily injury, but more than 100 were later diagnosed with traumatic brain injury.

The United States has blamed Iran-backed Kataib Hezbollah for a March 11 rocket attack that killed two American troops and a 26-year-old British soldier in Iraq and, a day later, carried out air strikes against its militants in Iraq.

Phillip Smyth, an expert who tracks Shi’ite militias at Washington Institute for Near East Policy think tank, said he believed Trump’s warning was prompted by the emergence of the League of the Revolutionaries, a group he said was formed to give deniability to Kataib Hezbollah to attack U.S. targets.

The United States and Iran have also been engaged in a war of words over U.S. sanctions, which aim to force Iran to curb its nuclear and missile programs as well as its use of proxies in conflicts in Iraq, Lebanon and Yemen.

Washington has repeatedly tightened its sanctions, which are designed to choke off Tehran’s oil exports, in the last month as the coronavirus outbreak has spread in Iran, one of the nations in the Middle East hardest hit by the virus.