The US and Britain have threatened Iran with new economic sanctions if it does not respond positively to incentives for Tehran to halt uranium enrichment. The US said the permanent members of the UN Security Council and Germany had agreed they would have no choice but to take further punitive measures. Britain said it would back sanctions if Iran failed to give what it called an unambiguous response by Tuesday. The move follows "inconclusive" talks between the EU and Tehran. Iran's chief nuclear negotiator, Saeed Jalili, spoke by phone to European Union foreign policy chief Javier Solana earlier on Monday. Mr Solana had given Iran until last Saturday to respond to an offer not to impose further sanctions against Iran in return for a freeze on its uranium enrichment programme. A spokesman for Mr Solana described the talks as "inconclusive". Mr Jalili said Iran would issue a formal written response to the offer on Tuesday.