Reports say the ship is carrying millions of rounds of ammunition
A Chinese ship carrying arms destined for Zimbabwe has been forced to leave the South African port of Durban four days after failing to unload.
Earlier, a South African judge ruled that the cargo of rocket-propelled grenades, mortar rounds and ammunition could not be transported overland.
Human rights groups had petitioned for a block on the arms and dockers had refused to unload the shipment.
Some fear Zimbabwe will use the arms to repress political opposition.
The country has yet to publish the results of its presidential election on 29 March, which the MDC opposition says was won outright by its candidate Morgan Tsvangirai.
President Robert Mugabe denounced the opposition on Friday in his first speech since the election, saying "thieves" were trying to steal the country.
According to the South African news agency Sapa, the ship upped anchor between 1800 (1600 GMT) and 1900 (1700 GMT).
The ship's master, who earlier identified himself as Captain Sunaijun, could not be reached by telephone, the agency added, quoting anonymous sources.
The transponder aboard the An Yue Jiang was not responding on Friday evening, the BBC's Adam Mynott reports from Durban.
The head of an independent human rights group monitoring the vessel said it was heading for Mozambique, en route to landlocked Zimbabwe.
Nicole Fritz, director of the Southern Africa Litigation Center, said her group aimed to pursue the issue with Mozambique.
Her group had called for the blocking of a permit allowing the arms to be offloaded from the An Yue Jiang and transported.
South Africa's government had said it could not legally prevent the arms being transported through the country but the high court in Durban ruled that the cargo could not be moved overland, though it could be discharged in the port.
'Nothing to do with us'
The ship contains three million rounds of ammunition for AK-47s, 1,500 rocket-propelled grenades and several thousand mortar rounds, according to South Africa's Mail and Guardian newspaper.
Aerial footage of the Chinese ship
The weapons are packed in large red and blue containers piled high on the ship's decks.
There are Chinese crew on board and the ship is flying both the Chinese and South African flags, our correspondent said after going to look at the ship anchored 18km (11 miles) from the entrance to Durban harbour.
The Mail and Guardian reports that a subsidiary of a state-owned South African company, Armscor, was approached to handle the transport of the weapons after several private companies refused to handle the cargo due to its sensitivity.
South African Defence Secretary January Masilela said the country's National Conventional Arms Control Committee had given approval for the transit of the weapons.
"If the buyer is the Zimbabwean sovereign government and the seller is the Chinese sovereign government, South Africa has nothing to do with that," he said.
Mr Masilela added that there was no United Nations or African Union embargo on weapons sales to Zimbabwe.
Speaking in New York, South African President Thabo Mbeki echoed his defence secretary's comments.
Several Western countries have banned arms shipments to Zimbabwe, as has the European Union.
With continuing tension in Zimbabwe over the failure of the authorities to issue results from the presidential election three weeks ago, the South African Transport and Allied Workers Union (Satawu) said it would be "grossly irresponsible" to allow the cargo through.
"The South African government cannot be seen as propping up a military regime," said Satawu General Secretary Randall Howard.
Zimbabwean opposition leader Morgan Tsvangirai said the weapons were not needed because Zimbabwe was not at war.
Zimbabwean Deputy Information Minister Bright Matonga insisted no country had the right to stop the arms entering his country.
For its part, China says the shipment is part of normal trade relations with Zimbabwe, adding that "one of the most important principles is not to interfere in the internal affairs of other countries".
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