Japan has reported a fresh suspected sanctions violation by Pyongyang to the UN after spotting an apparent cargo transfer involving a North Korean tanker, the Japanese foreign ministry said Tuesday.

The statement came after US President Donald Trump rolled out what he called the "heaviest sanctions ever" levied on the Pyongyang regime, designed to put the squeeze on North Korea's already precarious economy and fuel supply.

Tokyo said a military patrol plane observed the alleged transfer around 220 kilometres (136 miles) off Shanghai on Saturday night.

The Japanese plane found two tankers -- North Korea's Chon Ma San and the Maldive-flagged Xin Yuan 18 -- moored alongside each other on the high sea with their lights on, the foreign ministry said.

"Following a comprehensive assessment, the Government of Japan strongly suspects that they conducted ship-to-ship transfers banned" by the UN Security Council resolutions, the ministry said.

It was the fourth time this year Tokyo has reported a cargo transfer by a North Korean vessel in violation of UN sanctions over Pyongyang's banned nuclear and ballistic missile programmes.

Pyongyang is subject to a series of UN Security Council sanctions, including one prohibiting all member states from facilitating or engaging in ship-to-ship transfers of goods to or from North Korean-flagged vessels.

Trump has warned that if these sanctions do not work, "we'll have to go to phase two. Phase two may be a very rough thing." (AFP)