Washington (AFP) - Pentagon chief Chuck Hagel has castigated the US strategy in Syria in a memo to the White House, saying Washington must explain its intentions toward President Bashar al-Assad's regime.

The memo was sent last week to President Barack Obama's national security adviser, Susan Rice, a defense official said Thursday, confirming a New York Times report.

The memo was cited in the Times article by Hagel's aides as an example of how the Pentagon chief is more assertive behind the scenes than his reserved public performance might suggest.

Hagel warned that the Syria policy was "in danger of unraveling" due to confusion over the US stance toward Assad, the paper wrote.

The Obama administration has focused on defeating the Islamic State group in Iraq first, and described US-led air strikes in Syria as a way of disrupting the jihadists' supply lines.

Washington also plans to arm and train a group of 5,000 "moderate" Syrian rebels, but has not committed to attacking Assad regime forces that threaten moderate rebel fighters.

Some US lawmakers, analysts, retired military officers and Syrian opposition activists have heaped criticism on Obama's approach to Syria, accusing the administration of bolstering Assad's position by striking at IS without confronting the regime.

Anthony Cordesman of the Center for Strategic and International Studies, who has often advised the Pentagon, has slammed the US policy in Syria as a "strategic mess."