The United States stuck to its role as bad cop in the international standoff with a defiant Iran on Tuesday, calling Tehran's latest moves to gain nuclear technology a "serious escalation" that leaves the world few options. Over loud objections from around the world, Iran on Tuesday ended a two-year freeze on sensitive uranium enrichment activities. By resuming work on a program that can be used to make nuclear arms, Iran gave the US and Europe justification to haul it before the UN Security Council for possible sanctions. "If the regime in Iran continues on the current course and fails to abide by its international obligations there is no other choice but to refer the matter to the Security Council," White House press secretary Scott McClellan said. There is some confusion about exactly what activities Iran will pursue at a nuclear facility called Natanz, but McClellan said any resumption of enrichment and reprocessing activities would be "a serious escalation of the nuclear issue by Tehran."