NAIROBI, Oct 1 (Reuters) - The Sudan government on Thursday imposed a blanket ban on aid flights to the south of the country, according to the U.N.'s Operation Lifeline Sudan (OLS).

The government gave no reason for the decision and OLS said it hoped the ban was an administrative matter that would be quickly resolved through negotiation with the Khartoum government.

"There was a blanket denial today (Thursday) on flights anywhere in south Sudan," OLS spokeswoman Gillian Wilcox told Reuters. "No reason was given."

Aid workers are fighting to combat severe food shortages in parts of southern Sudan, mainly in the vast western region of Bahr el Ghazal.

Lindsey Davies, a spokeswoman for the U.N. World Food Programme, said: "We hope (the suspension) is not the first sign of something. We have 18 aircraft operating in well over 100 locations. It would have serious implications for our relief operation".

The government imposed a flight ban on Bahr el Ghazal between February 4 and 21, Wilcox said, in a measure that was widely criticised on the grounds that it prevented aid getting to an area facing a looming hunger crisis.

The government then relaxed the ban allowing flights to six locations before it was lifted entirely on April 1.

Aid workers say the ban comes at a bad time, particularly as a three-month ceasefire in Bahr el Ghazal is scheduled to end on October 15.

The ban also comes shortly after the Sudan People's Liberation Army (SPLA) launched its biggest offensive in 18 months when it attacked an area around the government-held town of Torit on September 16, according to aid workers and SPLA officials.

The Inter-Governmental Authority on Development, a regional grouping of seven states, is due to bring the SPLA and the government together in the Kenyan capital on October 7 to start talks on extending the ceasefire in Bahr el Ghazal.

OLS groups around 35 U.N. and non-governmental aid agencies. A few aid groups work outside OLS and are not limited by government restrictions.