A programme to train Syrian moderate rebels to take on Islamic State (Isis) costing $500m (£320m) has resulted in only "four or five" soldiers being successfully deployed, the top US military commander overseeing the conflict testified.

The strategy was central to US plans to retake territory controlled by the jihadist group in Syria, with the Pentagon announcing plans to train an army of more than 5,000 moderate rebels to take on IS on the ground in late 2014.

"The programme is much smaller than we hoped," conceded the Pentagon's policy chief, Christine Wormuth, before the Senate Armed Services Committee on 16 September. She added that between 100 and 150 fighters were currently undergoing training.

When quizzed on how many soldiers had thus far been successfully trained, General Lloyd Austin, commander of US Central Command, said: "We're talking four or five." He said there was not a large enough group to defend a safe zone near the Turkish border for civilians fleeing violence by IS and the forces of Syrian President Bashar al-Assad.

John McCain, the Republican Arizona Senator and former presidential candidate who chairs the committee, branded the Obama administration's strategy against IS a "debacle". Meanwhile, Senator Claire McCaskill, a Democrat from Missouri, mocked the training programme. She said: "It's time for a new plan."

The hearing comes after the Pentagon's inspector general launched an inquiry into allegations from intelligence officials that Pentagon chiefs are suppressing reports that expose the lack of progress in the US campaign against IS.

In August, Syrian rebel group Division 30, which includes US trained soldiers, declared it would not fight Al Qaeda allied rebel group IS following a series of targeted kidnappings by the jihadist group. Shortly after being deployed to Syria, the group's US trained commander and seven others were seized by Al Nusra. The group released seven of those kidnapped in August.