Berlin ? European foreign ministers said Thursday that nuclear talks with Iran had reached a dead end after more than two years of acrimonious negotiations and the issue should be referred to the U.N. Security Council.

The top diplomats from France, Germany and Britain, however, held back from calling for the 15-nation council to impose sanctions and said they remained open to more talks.

Iran’s top nuclear negotiator, Ali Larijani, told U.N. Secretary-General Kofi Annan that Tehran was interested in resuming “serious and constructive negotiations” with the Europeans but this time wanted a deadline.

Also Thursday, in an interview with CNN, Larijani said Iran wants to reach agreement with Europe and Russia but “the question of our research is non-negotiable.”

He also said that an offer to enrich uranium on Russian territory and then ship it back to Iran to fuel nuclear power stations “would be a good basis for negotiations.”