A senior U.S. official reportedly said Iran and North Korea have resumed cooperation on a long-range missile project.

Reuters reported on Sunday that the official said the project includes the "transfer of critical parts," but declined to say when such joint work first began, stopped and then started again.

The official reportedly said Iran could have sufficient fissile material for a nuclear weapon by the end of this year, without providing specific evidence to support the speculation.

There have been suspicions that Tehran and Pyongyang, allies against the United States, are cooperating in the development of ballistic missiles and nuclear technology.