The world?s big powers on Thursday sought to strike a common front on Iran?s controversial nuclear programme but failed to agree what steps to take if Tehran refused to meet their demands.

Meeting in Berlin, foreign ministers from the US, Britain, France, China, Russia and Germany emphasised a United Nations call for Iran to halt uranium enrichment, a process that can produce weapons-grade material. But Moscow underlined its opposition to sanctions, and was backed by Mohamed ElBaradei, head of the UN nuclear watchdog.