Iran vowed on Friday to avenge Israel’s killing of senior commanders and other officers of its elite Quds Force, at a public funeral held for the dead men, elevating fears of open war but leaving unsaid how it would retaliate or when.

U.S. officials in Washington and the Middle East said on Friday that they were bracing for possible Iranian retaliation for the Israeli airstrike on Monday in Damascus, Syria. U.S. military forces in the region have been placed on heightened alert. Israel has also placed its military on high alert, according to an Israeli official, canceled leave for combat units, recalled some reservists to air defense units and blocked GPS signals.

Two Iranian officials who asked not to be named because they were not authorized to speak publicly said that Iran had placed all its armed forces on full high alert and that a decision had been made that Iran must respond directly to the Damascus attack to create deterrence.

“Our brave men will punish the Zionist regime,” Gen. Hossein Salami, the commander in chief of Iran’s Islamic Revolutionary Guards Corps, told the crowd in Tehran attending the funeral of the officers killed in Damascus. “We warn that no act by any enemy against our holy system will go unanswered and the art of the Iranian nation is to break the power of empires.”

The Israeli airstrike hit a building that was part of the Iranian embassy complex in Damascus, killing three generals and four other officers of the Quds Force. The force, an arm of the Revolutionary Guards, conducts military and intelligence operations outside Iran, often working closely with allies that oppose Israel and the United States, including Syria, Hezbollah in Lebanon and Hamas.

Iran’s ambassador to the United Nations, Amir Saeed Iravani, said on Thursday that he would give interviews to U.S. news outlets “after Iran’s response to Israel.”