The United States could soon abandon its diplomatic approach to Iran's nuclear programme in favour of tougher action, a senior US official warned yesterday.

The major world powers of the UN Security Council permanent five plus Germany have been using a "dual track approach" of negotiations and punitive action to persuade the Islamic republic to stop enriching uranium.

"We had a dual track but it is less and less likely that we are going to be able to maintain that dual track and not move to the pressure track very soon," said Ellen Tauscher, US Under Secretary of State for Arms Control and International Security.

"We are running out of time to do what is right ... and to ease the tension in the region," she told reporters during a trip to Nato headquarters in Brussels.

Earlier US President Barack Obama pushed Tuesday for a "significant regime of sanctions" against Iran unless it accepts international proposals aimed at curbing its nuclear ambitions.

Stepping up the heat on Iran's leaders at an impromptu appearance in the White House briefing room, Obama said the United States and five other world powers were "moving along fairly quickly" to tighten the screws on Tehran.

He indicated that his administration had made headway in persuading Russia to overcome its traditional resistance to imposing new sanctions on Iran, even if he was uncertain about whether China would join the other powers.

China now appears to be the sole holdout on sanctions among the five veto-wielding members of the UN Security Council -- the other four are the United States, Russia, Britain and France.