By Megan Rowling

LONDON, March 18 (Reuters) - Sudan has assured the United Nations that it will not expel any more aid agencies working in the conflict-torn western region of Darfur, the U.N.'s top humanitarian official said on Wednesday.

Khartoum stopped 13 foreign and three local aid groups from operating in northern Sudan earlier this month saying they had collaborated with the International Criminal Court (ICC) in The Hague, which issued an arrest warrant for Sudanese President Omar Hassan al-Bashir on war crimes charges.

"We have received reassurances that there are no more expulsions in the pipeline -- at the same time, the president has said certain things about everybody has to respect the rules or they're out, which you need to take seriously," John Holmes, the U.N. under-secretary-general for humanitarian affairs, told Reuters in an interview.

Bashir warned at a rally on March 8 that the government would expel "anyone who goes against Sudanese law, whether they are voluntary organisations, diplomatic missions or security forces."

The president has also said he wants foreign agencies to hand over distribution of aid to Sudanese groups within a year.

But Holmes warned that, in some camps for displaced people, government officials were not welcome, and that could also be the case for some national aid agencies.

He said the United Nations hoped to have a discussion with the government "before too long" to obtain guarantees there would be no further expulsions and remaining organisations would be able to operate in a stable and secure environment.

The U.N. emergency relief coordinator said the Darfur aid operation - the world's largest - was not on the verge of collapse, but the remaining 72 international aid groups and the United Nations would struggle to make up for the removal of major agencies, such as Oxfam, Save the Children and two branches of Medecins Sans Frontieres.

"People always have coping mechanisms and we will try to fill the biggest gaps - but nevertheless, some very serious risks are being run by these kind of expulsions over time, as the effects of worse sanitation, unclean water, less medical surveillance, less medical availability starts to take its toll on the population," Holmes said.

The $1.05 billion aid operation planned for Darfur in 2009 would have to be re-examined following the reduction in agencies' ability to deliver relief, Holmes said.

He added that, while major aid donors had not indicated they would cut off funding yet, they might be wary of giving money to Sudanese organisations or aid agencies from other countries in the region which the government might bring in to help with the aid effort.

"Clearly they are watching and they're very concerned," said Holmes. "I think everybody's waiting to see."

(Reporting by Megan Rowling, editing by Richard Balmforth)

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