Talks have been stalled since February's summit between US President Donald Trump and North Korean leader Kim Jong-un ended without a deal due to differences over the scope of Pyongyang's denuclearization and sanctions relief from the US.

Asked if he believes North Korea is producing nuclear weapons, Stilwell told the Senate Foreign Relations Committee, "I assume they are."

Asked about the number of nuclear weapons, he said he would speak in a separate setting.

The International Atomic Energy Agency said recently that North Korea's main plutonium-producing reactor has been shut down long enough for it to have been refueled.

The 5-megawatt reactor at the Yongbyon nuclear facility has shown no signs of operations since early December, which is enough "for it to have been de-fuelled and subsequently re-fuelled," the IAEA said in a report submitted for its general conference this week.

North Korea can harvest enough plutonium for one nuclear bomb by reprocessing thousands of spent fuel rods that have been extracted from the reactor after about a year of operation.

Stilwell added that the Trump administration's policy is still the "full, verified denuclearization" of North Korea and that its "maximum pressure" sanctions campaign against Pyongyang continues. (Yonhap)