Britain’s Telegraph newspaper is reporting that U.K. officials fear that Iran or Russia may have helped North Korea’s “sudden advancement” in the development of nuclear weapons.

The paper on September 10 said the British Foreign Office is investigating whether “current and former nuclear states” helped Pyongyang develop nuclear weapons capable of being mounted on missiles.

“Iran is top of the list of countries suspected of giving some form of assistance, while Russia is also in the spotlight,” the paper reported.

Unidentified senior officials told the newspaper it was “not credible” that North Korea's scientists alone could have accomplished the nuclear advances.

“North Korean scientists are people of some ability, but clearly they’re not doing it entirely in a vacuum,” the paper quoted a U.K. government minister as saying.

North Korean on September 3 said it had successfully tested an advanced hydrogen bomb that can be loaded onto an intercontinental ballistic missile (ICBM), a move condemned by the United States, Russia, China, and most other world powers.

Based on reporting by The Telegraph