The European Union Aviation Safety Agency on Friday advised airlines to avoid Iranian airspace following threats of U.S. military action that have placed the Islamic Republic on heightened alert.

The regulator said the ongoing situation and the potential for U.S. attacks have increased the likelihood of misidentification by Iranian air defense forces.

It warned that the presence and possible use of a wide range of weapons and air defense systems, combined with unpredictable state responses and the possible activation of surface-to-air missile systems, create a high risk for civilian flights at all altitudes.

The advisory comes amid widespread unrest in Iran that began with protests over the rising cost of living on Dec. 28 and escalated into one of the largest protest movements against the clerical leadership since the Islamic Republic was founded in 1979.

At least 3,428 protesters have been killed, according to Norway-based Iran Human Rights, which described the figure as an absolute minimum and said more than 10,000 people have been arrested.

Until Wednesday, the United States had threatened military action if Iran did not abandon plans to execute detained protesters.

After warnings from Gulf allies about potential regional repercussions, Washington appeared to step back, though it reiterated Thursday that all options remain available.

U.S. President Donald Trump said Thursday he thanked the Iranian government for canceling all scheduled hangings after warning Tehran of serious consequences if the crackdown continued.