The UN's nuclear chief said Friday that Iran's pledge not to build a nuclear weapon would need "very strong" verification, as the United States and the Islamic republic negotiate a permanent settlement to end their war. Iran's nuclear programme is a key sticking point in talks to end the Middle East war, which began in late February with massive US-Israeli strikes. Iran and the US last week signed a preliminary deal to end the conflict, embarking on negotiations expected to address a host of disputes including the nuclear programme.