Iran’s joint military command warned Thursday that all oil tankers moving through the Strait of Hormuz must use its approved routes or face a “forceful response,” ratcheting up tensions again over a waterway crucial for international energy supplies.

The strait, the narrow mouth of the Persian Gulf, has emerged as one of the top issues in negotiations seeking a permanent end to the Iran war. The statement from the Khatam al-Anbiya military command, reported by Iranian state television, comes after both U.S. and Iranian diplomats met with mediators Wednesday in Qatar.

It wasn’t clear what sparked the threat from Iran. However, the U.S. military’s Central Command had put out a statement about a meeting with officials from Mideast nations in Bahrain that said “leaders underscored their shared commitment to the free flow of commerce through” the strait.