Russia, Syria, and Iran warned the United States against launching new strikes on Syria and called for an international probe into a chemical attack there earlier this month.

Russian Foreign Minister Sergei Lavrov, who hosted his Iranian and Syrian counterparts at a trilateral meeting in Moscow, called last week’s U.S. strike on a Syrian military base a “flagrant violation” of international law and warned that any further such action would entail “grave consequences not only for regional but global security.”

Syrian Foreign Minister Walid al-Muallem said the meeting sent a “strong message” to Washington.

Iran’s Mohammad Javad Zarif emphasized that the participants warned that any unilateral action by the U.S. is unacceptable.