US Secretary of State John Kerry urged Syria's opposition and regime to play a full role in peace talks Sunday, while accusing Bashar al-Assad's forces of starving civilians.

"This morning, in light of what is at stake in these talks, I appeal to both sides to make the most of this moment," he said, in an online statement broadcast from Washington.

While the top US diplomat aimed his remarks at both sides, his message was clearly aimed at the opposition, which has threatened to leave Geneva even before talks start.

The opposition High Negotiations Committee has demanded that humanitarian aid be allowed to flow to besieged towns before even engaging in indirect talks with Assad's envoys.

Kerry urged them to drop their preconditions, but also had hard words for their foes, accusing Assad's forces of deliberately starving the beleaguered cities.

"The town of Madaya is just an hour's drive from Damascus and yet its people have been reduced to eating grass and leaves," Kerry said.

"How have the regime and the militias that support it responded? By planting landmines and erecting barbed wire to keep relief workers out," he said.

Kerry added that Washington had received credible reports that another 16 people had starved to death in the rebel-held, regime-besieged community over the weekend.