Iranian regime was yesterday digging in for a confrontation with arch enemy the United States, with its supreme leader vowing not to halt a disputed nuclear drive despite looming UN Security Council action. US Secretary of State Condoleezza Rice said Iran, suspected by the West of seeking atomic bomb technology, might be the biggest challenge to US policy in the Middle East. Iran says it wants nuclear power only for civilian uses. Russia, anxious to avert any move to impose UN sanctions on Iran, urged Tehran to cooperate with UN nuclear inspectors.Speaking a day after it became clear the UN Security Council would take up the Iran standoff, Supreme Leader Ayatollah Ali Khamenei — ultimate decision-maker in the Islamic Republic — urged officials not to give in to Western pressure. “If the Iranian nation and government step back on nuclear energy today, the story will not end there and the Americans will make another pretext,” he told senior clerics. But he also called for “wisdom and expediency” in handling the issue, a possible nod to faint internal criticism in Iran that Iranian President Mahmoud Ahmadinejad and other senior officials have antagonized the West with needlessly inflammatory statements.