The US believes Russia may be conducting low-level nuclear testing in violation of a moratorium on such tests, the head of the Defense Intelligence Agency says.

"The United States believes that Russia probably is not adhering to its nuclear testing moratorium in a manner consistent with the 'zero-yield' standard," Lieutenant General Robert P Ashley said at an arms control forum at the Hudson Institute on Wednesday.

In this video grab provided by RU-RTR Russian television via AP television on Thursday, March 1, 2018, the Avangard hypersonic vehicle blasts off during a test launch. (AAP)

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.

Russian President Vladimir Putin this week. (AAP)

Russia ratified the treaty in 2000.

"We believe they have the capability in the way they are set up" to conduct low-level nuclear tests that exceed the zero yield limit set in the CTBT, Ashley said.