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. But there has been contrasting information from Tehran and Washington on whether UN inspectors will have access to Iranian nuclear facilities. Watch more'We will have to go back to a negotiating table': IAEA chief Grossi on Iran's nuclear capabilities "I think the objective of this agreement is to ensure that there is no development of nuclear weapons in Iran. The government of Iran has declared quite clearly that this is not their intention," International Atomic Energy Agency (IAEA) head Rafael Grossi said Friday. "But of course intentions are not enough. We have to have a very strong verification system in place... as soon as is practicable," he added. Grossi said the UN's nuclear watchdog had so far "barely initiated" talks with Iran following the agreement with the United States. The deal specifies that the country's stockpile of enriched uranium should be "downblended" under IAEA supervision.