The United Nations has suspended aid flights in the upper Nile region of southern Sudan because of rapidly increasing insecurity.

Four people from a UN light aircraft are facing their fifth night of captivity having been seized by pro-government militia on Thursday.

Large numbers of people in the region have been forced from their homes as rival factions battle for position and there are warnings of a new humanitarian emergency.

Flights have been suspended because, in the words of one UN official, "we don't know who'll be in charge when we land".

The four who are on board the aircraft which has been held since Thursday have been enmeshed in a complex and volatile situation.

Oil wealth

Last year, quantities of oil started to be pumped from Upper Nile. The reality of so much wealth leaving the south for the north has fundamentally altered alliances.

As rival factions were wooed by both the rebels and government, the regime in Khartoum continued a cynical policy aimed at keeping the communities divided.

Its former southern ally Riek Macher, has resigned from the government and is said to be in upper Nile.

It is some of his former followers who are holding the aircraft.

Humanitarian consequences

The factional fighting has had a disastrous effect on the general population.

Because of flight restrictions imposed by the government and insecurity, the World Food Programme last year estimated it reached just 30% of the 280,000 people needing help.

The UN report said the situation in Upper Nile could rapidly develop into an acute crisis comparable to the famine of 1997.

A lengthy flight ban by the UN and affiliated aid organisations will remove international observation at a crucial time.