U.S. President Donald Trump on Wednesday warned Iran and its proxies against attacking U.S. troops or assets in Iraq, citing a possible “sneak attack” but giving no other details.

“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. (1600 GMT) intelligence briefing.

At odds for decades, the United States and Iran have seen relations deteriorate further in the nearly two years since Trump abandoned Iran’s 2015 multilateral nuclear deal and reimposed U.S. economic sanctions on the Islamic Republic.

The United States blamed an Iran-backed militia 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 the Kataib Hezbollah militants in Iraq.