Rex Tillerson, the US secretary of state, warned Saudi Arabia and Iran yesterday not to use Lebanon as “a venue for proxy conflicts” amid fears that their power struggle would destabilise the small country and cause fresh chaos in the region.

One week after after Saad Hariri, the Lebanese prime minister, abruptly resigned during a trip to Saudi Arabia questions continued to mount over whether he was being effectively held prisoner in Riyadh.

The tensions over Mr Hariri’s fate and other issues prompted the United Nations secretary general to appeal for calm and warn that a fresh conflict in Lebanon would have “devastating consequences” for the Middle East.

“The United States supports the stability of Lebanon and is opposed to any actions that could threaten that stability,” Mr Tillerson said in a statement. “The United States cautions against any party, within or outside Lebanon, using Lebanon as a venue for proxy conflicts or in any manner contributing to instability in that country.”

Without naming specific countries or groups, he said all regional players must “respect the integrity and independence” of the Lebanese government and military and called Mr Hariri “a strong partner of the United States”.