New York (AFP) - US Secretary of State John Kerry suggested Tuesday that Russia and Iran may persuade Syrian strongman Bashar al-Assad to stop indiscriminately dropping barrel bombs on his own people.

Moscow and Washington disagree sharply about Assad's longer term role in efforts to end the Syrian war, but have begun cautiously to discuss efforts to defeat his Islamic State jihadist foes.

In an interview with MSNBC News on Tuesday, Kerry suggested that the cooperation might go deeper than that and that Moscow and Tehran, Assad's staunch allies, might also help rein him in.

He said that he had discussed the matter with Iranian officials and his Russian counterpart Sergei Lavrov, whom he is to meet again this week on the sidelines of the UN General Assembly.

"And we raised that yesterday," he said. "I did talk with Lavrov about it and I talked with the Iranians about it."

Kerry went on to say "they are both in a position, in exchange perhaps for something that we might do, they might decide to keep Assad from dropping barrel bombs."

The top US diplomat did not say what he might offer Moscow or Tehran in exchange for halting the bombing, but his comment came as both were being drawn into efforts to fight the Islamic State.

Assad's barrel bombs have become emblematic of the cruelty of his campaign to suppress a revolt against his rule -- crude explosive devices packed with shrapnel and dropped from helicopters.

Western powers argue that such tactics have cost Assad his right to rule, but Moscow and Tehran say he remains Syria's legitimate leader and a bulwark against jihadist rebels.