A Chinese cargo ship believed to be carrying 77 tonnes of small arms, including more than 3m rounds of ammunition, AK47 assault rifles, mortars and rocket-propelled grenades, has docked in the South African port of Durban for transportation of the weapons to Zimbabwe, the South African government confirmed yesterday. It claimed it was powerless to intervene as long as the ship's papers were in order.

Copies of the documentation for the Chinese ship, the An Yue Jiang, show that the weapons were sent from Beijing to the ministry of defence in Harare. Headed "Dangerous goods description and container packing certificate", the document was issued on April 1, three days after Zimbabwe's election. It lists the consignment as including 3.5m rounds of ammunition for AK47 assault rifles and for small arms, 1,500 40mm rockets, 2,500 mortar shells of 60mm and 81mm calibre, as well as 93 cases of mortar tubes.

The carrier is listed as the Cosco shipping company in China.

South Africa's national conventional arms control committee issued a permit on Monday for the trans-shipment of the cargo from Durban to Harare. The head of government information in South Africa, Themba Maseko, said yesterday: "We are not in a position to act unilaterally and interfere in a trade deal between two countries." South Africa had to "tread very carefully", given the complexity of the situation in Zimbabwe, Maseko said.

South Africa was not encouraging the purchase of weapons by Zimbabwe, he said, pointing out that there was no UN trade embargo against that country.

But Tony Leon, the South African opposition foreign affairs spokesman, said the shipment was tantamount to "putting a fuse in a powder keg".

Dockers in Durban were refusing last night to unload the ship. The SA Transport and Allied Workers Union's general secretary, Randall Howard, said: "Satawu does not agree with the position of the government not to intervene with this shipment of weapons. Our members will not unload this cargo, neither will any of our members in the truck-driving sector move this cargo by road."

Despite international criticism, the Chinese government has been a longstanding backer of Zimbabwe president Robert Mugabe's authoritarian regime, supplying it with jet fighters, military vehicles and guns. China, or Chinese businesses, are reported to have sold radio-jamming devices to prevent independent stations from contradicting the state-controlled media, and have signed vital agriculture deals. Even the blue tiles on Mugabe's latest 25-bedroom mansion, reminiscent of Beijing's Forbidden City, were a gift from China.

China has in the past used its veto at the UN security council to prevent the Zimbabwe issue from being raised, on the grounds that the country's problems were an internal matter.

In Britain, William Hague, the shadow foreign secretary, said last night: "The international community must speak with one voice on Zimbabwe. We call on China, as part of that community, to suspend arms sales to Zimbabwe.

"The Mugabe regime continues to deny the right of the people of Zimbabwe to choose their leaders. To supply arms to it at time when opposition activists are being intimidated and attacked, not only sends the wrong signal, but will harm the reputation of China.

"In addition, it is time that neighbouring states like South Africa made clear that such shipments are not welcome."

The Foreign Office was more cautious. A spokeswoman said that Britain backed an EU ban on arms sales to Zimbabwe and was encouraging other governments to do the same. The FO said it was monitoring the situation and seeking to verify reports about the ship's cargo.

A spokesman for China's foreign ministry said it was aware of the reports about the shipment, but needed more time to look into the matter.

The disclosure about the ship's cargo follows claims by an official from the Zimbabwe opposition Movement for Democratic Change that Chinese soldiers had been seen in the country.

There were some signs yesterday that South Africa may at last be bending under international pressure, when the cabinet joined calls for the release of Zimbabwe's election results.

Zimbabwe's opposition leader, Morgan Tsvangirai, called on South Africa's president, Thabo Mbeki, to stand down as the chief mediator in the country's election crisis, as the US criticised African governments for lack of action on the issue. "It is time for Africa to step up," the US secretary of state, Condoleezza Rice, said.

Tsvangirai told a news conference in Johannesburg: "President Mbeki needs to be relieved from his duty."

Mbeki, is also under pressure from Jacob Zuma, the leader of the ruling African National Congress. Zuma has adopted a more hostile attitude towards Mugabe, saying that "the region cannot afford a deepening crisis in Zimbabwe".