The United Nations and the government of Sudan are working to secure the release of a World Food Program aircraft seized by pro-government fighters in southern Sudan.

They are reported to have taken four people on board hostage while demanding the release of prisoners taken by anti-government rebels.

Newspapers in Khartoum identified those on board as an American working for the United Nations, a Sudanese national from southern Sudan and two Kenyan pilots.

A statement from the Sudanese Government's Humanitarian Aid Commission said: "We regret the occurrence of such an incident and would like to say that we have already begun coordinating with the UN office in Khartoum to try to end this crisis as soon as possible."

The pro-government militiamen are seized the plane on Thursday in the Panjak region of southern Sudan.

A WFP spokesman in Khartoum said the plane was hijacked in Fanjak, about 750km (470 miles) south of Khartoum.

It had been flown in from Lokichokio in Kenya.

WFP said the two-man crew and the UN official on board were in touch with the United Nations and said it hoped the crew and plane would be released as soon as possible.

Retaliation

The pro-government Akhbar al-Youm newspaper said the plane was being held by a pro-government forces leader Gabriel Tanginya.

He is believed to be angry because another WFP plane had flown three of his commanders to Fanjak where they were captured by the rebel Sudan People's Liberation Army.

Sudan's government has said it regretted the action by its supporters and insisted it was committed to delivering food and supplies to the needy in Southern Sudan.

Sudan's civil war pits the Islamist Khartoum government against the mainly Christian and animist southern rebels.

An estimated 1.5 million people have died since the war between the Khartoum government and rebels began in 1983.

They are reported to have said the UN representatives would not be released until their leaders are freed.

The UN-sponsored Operation Lifeline-Sudan started delivering delivering food and other relief supplies to people in war-torn areas of south Sudan 12 years ago.