By Hereward Holland

JUBA (Reuters) - The United States is urging Sudanese rebels to stop recruiting soldiers from among children and other civilians in refugee camps across the border in South Sudan, a U.S. official said on Wednesday.

Fighting in Sudan's southern border states has raged for more than a year, complicating peace talks with newly-independent South Sudan and prompting aid agencies to warn of an impending humanitarian catastrophe in the region.

Hundreds of thousands of people have fled their homes in Sudan's South Kordofan and Blue Nile states since clashes between government forces and the rebel Sudan People's Liberation Movement North (SPLM-N) broke out over a year ago.

Anne Richard, the U.S. assistant secretary of state for population, refugees and migration, said she had asked the SPLM-N to stop enlisting soldiers from South Sudan's Yida camp, which houses more than 60,000 Sudanese refugees.

"(We) have asked them to please not use the camp, which is supposed to be civilian, as a centre for R&R (rest and relaxation) or for recruitment of soldiers," Richard told reporters after visiting Yida.

"Especially we've asked that they not take children to serve as soldiers on the other side of the border."

For all its mild wording, the U.S. request casts doubt on Juba's repeated denials that it is helping the SPLM-N, a force they fought alongside in the country's 1983-2005 civil war.

Washington, which backed the peace process that led to South Sudan's secession and maintains trade sanctions on Sudan, has only rarely criticized the new country's government.

South Sudan broke away from Sudan last year under the peace deal that ended the decades-long conflict.

REBELS DENY CHARGES

The United Nations says more than 175,000 Sudanese refugees have sought sanctuary in South Sudan since fighting began in South Kordofan and Blue Nile.

Last month South Sudan and Sudan signed deals to resolve border and security issues left over after partition, and promised to halt support for rebels in each other's territory.

In a visit to Yida refugee camp this week Richard said she saw uniformed military men, but did not confirm who they were.

"That's not the way refugee camps for civilians are meant to be run," she said. "We had reports at the mid to end of September, reports that recruitment was going on (although) my understanding is that some of that has subsided."

SPLM-N spokesman Arnu Lodi denied reports of recruitment.

"All our troops are inside Sudan. If they come to Yida they are visiting their families and they leave their weapons outside," he said.

Richard said malnutrition in the camp had dropped in recent months but warned tens of thousands of people may come to the camps as seasonal rains dwindle towards the end of the year.

Khartoum has denied aid agencies access to rebel-held territory since the conflict began, fearing the aid would fall into the hands of the insurgents.

(Editing by Alexander Dziadosz and Alistair Lyon)