South Africa's leading opposition party on Thursday challenged the government to explain why a Russian cargo ship that it said was under western sanctions had docked at a Cape Town naval base.

The vessel arrived at Simon's Town -- South Africa's largest naval base -- on Tuesday, the Democratic Alliance (DA) party said, adding it seemed to have turned off its automatic identification system, which provides the ship's position and other information.

At night, cranes had been offloading cargo from it and onto trucks protected by armed personnel, it added.

"This behaviour has local residents worried because the vessel is under sanctions by the US and European Union after Russia invaded Ukraine," shadow defence minister Kobus Marais said in a statement.

One of the continent's power houses, South Africa has resisted taking sides following Russia's invasion of Ukraine, a war which has triggered sweeping Western sanctions.

Marais urged the defence minister to explain why the ship docked at a military rather than a commercial port and "why there is so much secrecy surrounding it".

The defence ministry and the navy did not immediately respond to a request for comment.

The ship, identified by local media as the Lady R, flies a Russian flag and is en route from Cameroon to Tanzania, according to vessel tracking websites.

Earlier this year, the South African government was criticised for not refusing entry to a luxury megayacht linked to a sanctioned Russian oligarch close to President Vladimir Putin.

The $500 million vessel eventually did not dock in the country as expected.