The United States believes Russia may be conducting low-level nuclear tests, a U.S. intelligence official on Wednesday, while the head of a body monitoring a global nuclear treaty said there was no sign of such violations by Moscow.

Negotiated in the 1990s, the Comprehensive Nuclear Test Ban Treaty (CTBT) enjoys wide global support but must be ratified by eight more nuclear technology states, among them Israel, Iran, Egypt and the United States to come into force.

Russia ratified it in 2000.