DUBAI, United Arab Emirates (AP) — Iran and the United States held a fourth round of negotiations Sunday over Tehran’s rapidly advancing nuclear program, just ahead of a visit by President Donald Trump to the Middle East this week.

Fresh talks between Iranian and U.S. negotiators to resolve disputes over Tehran's nuclear programme ended in Oman on Sunday with further negotiations planned, officials said, as Tehran insisted in public on continuing uranium enrichment.

Iran described the latest meeting as "difficult but useful," while a senior U.S. official, speaking to AFP on condition of anonymity, said Washington was "encouraged by today's outcome and look forward to our next meeting,