Amid growing concerns over military cooperation between North Korea and Russia, renowned nuclear scientist Siegfried Hecker said Moscow could secretly and directly provide plutonium to assist in Pyongyang's nuclear arms development.

In an interview with Joel Wit, senior fellow at the Stimson Center and director of U.S.-based website 38 North, on Thursday, Hecker said his biggest concern in the short term is the clandestine and direct supply from Russia of its stock of around 125-thousand kilograms of plutonium from the Soviet era.

The scientist said there would be no technical hurdle to ship 100 or even one-thousand kilograms to the North, which would allow the regime to "exponentially" increase its nuclear arsenal.

In the long term, Hecker said Russia could help the North operationalize its experimental light water reactor, which could then be repurposed for plutonium production, while justifying the assistance as efforts to establish peaceful electrical power generation.

Moscow could also supply fresh fuel for the IRT-2000 reactor, built in the 1960s with support from the Soviet Union, which could provide a small amount of plutonium and even tritium, fuel for hydrogen bombs.

Hecker also talked about Russia's possible sharing of nuclear weapons designs and experimental data to accelerate the North's efforts to gain the ability to mount a nuclear warhead on an intercontinental ballistic missile and deliver it to the mainland U.S.