Robert Mugabe, the president of Zimbabwe, is secretly arming Laurent Gbagbo, whose refusal to accept defeat in the presidential election in the Ivory Coast has brought his west African country to the brink of war.

A giant chartered Antonov An-22 cargo plane with Angolan registration delivered tons of weapons from Harare to Gbagbo over Christmas and the new year, highly placed intelligence sources in Zimbabwe’s capital revealed last week.

The aircraft took off from Manyame airbase outside Harare. The exact quantity of arms is not known but the Soviet-built Antonov can carry up to about 80 tons of cargo. Zimbabwean military and intelligence officials accompanied the weapons on the flight.

Earlier, the sources said, the plane had flown into Manyame with a consignment of small arms, mortars and rockets from China – Mugabe’s chief arms supplier – for the Zimbabwean army.

On Mugabe’s instructions, part of this shipment remained on board and was supplemented with more armaments from the stocks of Zimbabwe Defence Industries, the state arms maker. A few hours later the plane flew to the Ivory Coast where the cargo was secretly unloaded.

Sources in Harare said that Mugabe, 86, had authorised the arms shipment after an appeal from Gbagbo for military assistance in return for oil. The sources said that a mysterious Chinese businessman – identified only as Sam Pa – had played a pivotal role in organising the shipment so that it could not be traced back to Mugabe.

Sam Pa uses a variety of aliases. His main business interests are in oil in Angola but he has lately expanded into diamond-rich Zimbabwe, where he has established commercial relations with some of the most powerful figures in Mugabe’s inner circle.

The clandestine arms delivery pits Mugabe against the United Nations, west African leaders and the African Union. The UN has 10,000 peacekeepers in the Ivory Coast and has had an arms embargo in force since 2002.

International pressure is mounting on Gbagbo to hand over power peacefully to his rival, Alassane Ouattara, the would-be president, who won last November’s presidential election run-off, according to UN-verified results.

Economic sanctions and diplomatic measures are favoured to get Gbagbo to step down but force has not been ruled out as a last resort.

In the Ivory Coast, the army is the one part of the state machinery that has remained intensely loyal to the beleaguered Ivorian leader throughout the crisis.

Arrangements for face-to-face talks between the two rivals have twice failed because the army has refused to lift a blockade around a luxury hotel in Abidjan, the commercial capital, where Ouattara is holed up. If fighting does break out, the arms sent by Mugabe could be a crucial boost for the troops willing to try to keep Gbagbo in power.