MELBOURNE, Australia—Some of the hundreds of refugees being held in Australia-backed Pacific island camps are to be resettled in the U.S. under a one-time deal between the countries.

The U.S. government has agreed to accept refugees being held in Nauru and Papua New Guinea’s Manus Island, Prime Minister Malcolm Turnbull said Sunday. Mr. Turnbull didn’t disclose how many were likely to resettled by the U.S. or on what terms, but he stressed the arrangement wouldn’t be repeated or be extended to asylum seekers not already in...