WASHINGTON - The six world powers negotiating with Iran would consider offering limited sanction relief if Iran takes concrete steps to halt its nuclear activities, the White House said Thursday.

The six countries, namely the US, Russia, China, Britain, France and Germany, would consider "limited, targeted and reversible relief" that does not affect their core sanctions architecture in exchange for concrete, verifiable measures to address their concerns about Iran's nuclear project, said White House spokesman Jay Carney at a daily briefing.

"That core sanctions architecture would be maintained until there is a final, comprehensive, verifiable agreement that resolves the international community's concerns," said Carney, who also warned that if Iran does not live up to its commitments, relief would be terminated, and the world powers would ratchet up the pressure even further by adding new sanctions.

Delegates of the six world powers, known as the P5+1 group, started another round of nuclear talks with Iran in Geneva, Switzerland, Thursday.

In the two-day talks, the world powers are expected to seek a "first step" deal with Iran on solving the nuclear problem, though both sides say an agreement is far from certain.

In the previous round of talks, held on October 15-16, Iran proposed a plan as a basis for negotiation but details of the proposal were not disclosed.