Four people who are on board a United Nations aircraft which was seized in southern Sudan have now been held for three nights, while negotiations to secure their release are continuing.

The four, who include a United States official, a south Sudanese, and two Kenyan pilots have been detained by pro-government forces.

The United Nations is negotiating with the government in Khartoum, and so far the seizure has not affected the flying of aid into government and rebel held areas of southern Sudan.

But the longer the situation continues the more likely it is that relief operations will be reviewed.

The four hijack victims have used a satellite telephone to communicate, but its battery is running low.

Humanitarian problems

The light aircraft was seized in Upper Nile province, an area that faces acute humanitarian problems.

The government has recently started to pump oil from the area and the visible flowing of wealth from the south to the north has fundamentally changed alliances and attitudes.

At the weekend it was confirmed that Riek Machar, whose South Sudan Independence Movement was allied to the government, has resigned his ministerial post in Khartoum and is now thought to be in Upper Nile.

Those holding the light aircraft used to be followers of Machar and are still described as pro-government.

But in the southern Sudanese civil war, the government has promoted ethnic division, supported rival commanders and encouraged chaos.

In such a situation it is sometimes difficult to tell how much control Khartoum exercises over those to whom it gives arms.