JUBA, Sudan (Reuters) - The parliament of South Sudan has approved a supplementary budget equivalent to more than 60 percent of the original total for 2008, mainly to cover overspending by the army of the semi-autonomous region.

At a session late on Thursday the parliament agreed to add 2.08 billion Sudanese pounds to the 3.4 billion pounds it had already approved for the calendar year.

Kuol Athian, finance minister of the southern region, which has a special status under the 2005 peace agreement between north and south, said the money was needed to cover expenditure by the army, which spent all its original budget by July.

The former rebels who dominate the South Sudan government retained their guerrilla forces after the peace agreement, converting them into an autonomous army.

Diplomatic sources say that both the northern based government and the southern leaders are covertly stockpiling weapons for fear that flashpoint events in coming years could plunge them back into conflict.

The chairman of parliament's economic committee, Bari Wanji, said of the supplementary budget request: "We have hardly any choice but to pass this."

Speaking in the house, he waved a copy of the independent Sudan Mirror newspaper showing a fighter jet which it said the Khartoum government had bought.

Wanji said one cause of insecurity in the south was the Ugandan rebel Lord's Resistance Army, which is active near the border with Democratic Republic of Congo. Other lawmakers said failing to pay the soldiers in the south's swollen army would be an enormous security risk.

Athian told the parliament last week that especially high oil prices in the first half of 2008 meant the south could afford the budget hike but that lower revenues now would mean belt-tightening next year.