Iran must take up President Barack Obama's offer of new dialogue on its disputed nuclear program or face tougher sanctions, British Prime Minister Gordon Brown said Tuesday. Brown, speaking to a London conference, said Iran had become a "critical proliferation threat" because of its refusal to heed international concern over its uranium enrichment program, according to excerpts of his speech released in advance. Brown said Iran should face automatic penalties for breaching the international rules that cover its nuclear program - a move that would make the sanctions process quicker. Punishments currently have to be negotiated at the United Nations Security Council, where Russia and China often block tough action against Iran. The threat of stronger punishment would complement Obama's call for dialogue, Brown's speech said.