Iran must take up US President Barack Obama's offer of new dialogue on its disputed nuclear program or face tougher sanctions, British Prime Minister Gordon Brown said Tuesday. Speaking to a London conference, Brown 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. While the US and others claim Iran is enriching uranium as part of a quest to develop nuclear weapons, Iran insists the program is part of a peaceful civilian energy program. 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. "I hope that Iran will make the right choice and take advantage of the international community's willingness to negotiate, including President Obama's offer of engagement, rather than face further sanctions and regional instability," the speech said. The Obama administration has repeatedly raised the prospect of diplomatic engagement with Iran, but the offer has met with a mixed response from the Islamic Republic. Sanctions have also failed to slow its nuclear activities: US and Israeli officials believe the country had enough fissile material to produce an atomic bomb.