DUBAI, United Arab Emirates — Iran and the United States began a fourth round of negotiations Sunday over Tehran’s rapidly advancing nuclear program , officials said, 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 publicly insisted on continuing its uranium enrichment.

Tehran called Sunday’s meeting “difficult but useful” as the two sides clash over future of Iran’s uranium enrichment.