Iran vowed 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 Friday that they were bracing for possible Iranian retaliation for the Israeli airstrike Monday in Damascus, Syria. U.S. military forces in the region have been placed on heightened alert. According to an Israeli official, Israel has also placed its military on high alert, 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.