Anne Richard, US Assistant Secretary of State for the Bureau of Population, Refugees and Migration, said Washington had called on rebels fighting in Sudan's South Kordofan and Blue Nile states to end militarisation of the Yida refugee camp.
"We have asked them not use the camp, which is supposed to be civilian, as a centre for R&R (rest and recuperation) or for recruitment of soldiers," Richard told reporters after a visit to Yida, in South Sudan's Unity state.
"Especially, we've asked that they not take children to serve as soldiers on the other side of the border," she added, speaking in South Sudan's capital.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The Sudan People's Liberation Movement-North (SPLM-N) have been fighting government forces since June 2011 for greater autonomy.
The insurgents were allies of then southern rebels during Sudan's 1983-2005 civil war, which ended with a peace deal that led to South Sudan's independence in July last year.
Sudan has repeatedly accused former civil war enemies in South Sudan of supporting the SPLM-N and allowing the insurgents to set up rearguard bases, a charge which analysts believe despite denials by the government in Juba.
South Sudan declined to comment Wednesday on Richard's comments, the first time a senior Western official has said the SPLM-N are operating inside South Sudan.
On its part, the SPLM-N has denied recruiting child soldiers, terming the accusations by the US as "false information that does not have any reality on the ground."
"SPLM-N does not recruit any child soldiers...it is not true...we need an investigation to go and look for these child soldiers," SPLM-N spokesperson Arnu Ngutulu told AFP.
Richard said the overcrowded Yida camp -- home to over 60,000 refugees from Sudan who have fled hunger and aerial bombardment -- should be moved further south and away from the border.
"The Yida camp was bombed a few months ago, there are a lot of movements in that border area of military combatants," she said.
"According to international standards, refugee camps are supposed to be run for and by civilians," Richard explained, saying that they had seen "uniformed military in the camp".
The SPLM-N however says it has no presence at Yida camp, saying the group does not operate in South Sudan.
"Yida refugee camp is inside South Sudan and SPLM-N is not on that soil...we are in Sudan," Ngutulu said.
However, both rebels and South Sudanese soldiers often wear similar mismatched uniforms, and it can be hard to determine what force men belong to.
Insurgents have reported an upsurge in fighting since Sudan and South Sudan signed a deal in September for a demilitarised border buffer zone hoped to cut support for the rebels.
The pact was among agreements on security and oil, sealed with African Union mediation, which leaders of the two nations hailed as ending their conflict.
Richards said that South Sudanese officials had recognized the problem too, and that recruitment was thought to have "subsided" since high levels in September.
However, Washington wants "mothers and the children moved to a much safer location where we can provide a lot more education and skills training."
Over 900,000 people have been affected by the conflict in Blue Nile and South Kordofan, according to estimates by the UN Office of Humanitarian Affairs (OCHA).
But despite more than a year of talks food aid is still blocked from entering rebel zones where dire food shortages are reported, the UN has said, with more expected to flee in the months ahead.
With the dry season approaching -- traditionally heralding an increase in violence as army vehicles can move more easily -- "there could be tens of thousands" fleeing battle zones and depleting scarce resources, Richards added.
