The United States said it is studying a UN sanctions report alleging North Korea and Iran are sharing ballistic missile technology.

"It raises concerns, but concerns that we've had in the past," State Department spokesman Mark Toner told reporters. "We're looking at the report and we're studying it."

Diplomats said a Chinese representative on a UN panel of seven experts on North Korea nuclear sanctions refused to sign on the report, which said prohibited material was moved through a "neighboring third country."

The country is not named in the report, excerpts of which were passed to AFP, but diplomats said it was China, the isolated north's closest ally.

The New York Times reported that Beijing had put political pressure on the expert not to sign.

"There are certainly going to be some tough talks on the report this week," said one diplomat, speaking like the others on condition of anonymity. The sanctions panel's work is to be discussed on Tuesday.

The UN Security Council imposed sanctions against North Korea after two nuclear tests in 2006 and 2009. It is banned from dealing in nuclear and ballistic material.