Two Russian ships made at least five round trips between North Korea and Russia, beginning mid-August, in what could be arms transfers, the Washington Post reported Monday, citing satellite imagery.

Based on analysis by the Royal United Services Institute, a London-based think tank, the ships have been travelling between the northeastern North Korean port of Najin and a port facility in Dunay in Russia's Far East between mid-August and Saturday.

The analysis came after the US government revealed Friday that the North shipped more than 1,000 containers of military equipment and munitions to Russia in recent weeks for use in Ukraine, highlighting burgeoning military cooperation between Pyongyang and Moscow.

According to the news report, the apparent transfers began in mid-August -- about three weeks after Russian Defense Minister Sergei Shoigu visited Pyongyang and met with North Korean leader Kim Jong-un.

It was also about a month before Kim held talks with Russian President Vladimir Putin at the Vostochny Cosmodrome, a Russian spaceport on Sept. 13.

The White House mentioned only one Russian-flagged vessel, MV Angara, involved in the transfers between Sept. 7 and Oct. 1. But according to the Washington Post, MV Maria was also spotted in the satellite imagery.

Although it is impossible to verify what the ships' cargo was, the satelite images showed that the ships are linked to the Russian military logistics network, which suggests they were carrying military equipment, RUSI analysts and US officials were quoted by the news outlet as saying.

Shortly before the operation began, the two vessels turned off their automatic identification systems to hide their movements, the newspaper said, citing data from Marine Traffic, a ship-tracking site.

Seoul and Washington have said that any arms transfers between the North and Russia would be in violation of multiple UN Security Council resolutions and a challenge to global peace. (Yonhap)