Rival troops build up in Sudan's Abyei -satellite
* Abyei seen as flashpoint ahead of southern secession
* UN concerned over army, militia clashes in south's Jonglei
KHARTOUM, March 11 (Reuters) - Troops linked to north and south Sudan have taken up new positions in the contested Abyei region, according to satellite monitoring, raising fears of an escalation of violence.
Analysts say fertile and oil-producing Abyei is one of the most likely places for conflict to erupt in Sudan as the country prepares for the secession of its south, due on July 9.
More than 100 people died in clashes between groups aligned to either side last week, the southern army said. Washington said on Wednesday the violence was "unacceptable" and urged northern and southern leaders to reach a quick deal.
Southerners, who mostly follow Christian and traditional beliefs, voted overwhelmingly to declare independence in a January referendum promised in a 2005 peace deal that ended decades of civil war with the Muslim north.
Residents of the Abyei border region were promised their own vote on whether to join the north or south on the same day.
But that vote never took place due to disagreement between northern Arab Misseriya nomads and the south-linked Dinka Ngok group over who was qualified to vote.
Satellite images showed armed men had moved to new frontline positions in Abyei where they had built up forces and dug in, according to a statement from the Satellite Sentinel Project, set up by actor George Clooney and other activists.
Southerners have accused the north of arming the Misseriya and using its allied Popular Defence Force militia to raid inside the Abyei area and burn down thee villages last week.
The north's army and ruling National Congress Party (NCP) have dismissed the accusations and accused the south of sending in soldiers disguised as police officers -- an allegation denied by the south.
"This increased military activity is a warning that the recent village razings in Abyei could be a prologue to wider and worsening conflict," said John Bradshaw, executive director of the Enough Project campaign group, part of the satellite project.
"Northern-aligned troops appear to have constructed a forward operating base in the past two weeks at Bongo, some 15 km (9 miles) from the recently razed village of Maker Abior," said the statement from the satellite group.
"The Bongo base is some 20 km to the north of where SPLA (southern army)-aligned forces appear to have trenched in at the razed villages of Todach and Tajalei," it added.
No one was immediately available for comment from the northern or southern armies on Friday.
The United Nations on Thursday said it was growing increasingly concerned about a separate southern military operation against rebel militias in the south's Jonglei state -- the site of a oil field controlled by France's Total (TOTF.PA: Quote, Profile, Research, Stock Buzz).
In recent weeks, south Sudan's army has clashed several times with militia fighters loyal to renegade southern leader George Athor who rebelled last year saying he had been cheated out of the governorship of Jonglei in elections.
The United Nations called on the southern army on Thursday to let U.N. groups into the area and to avoid harming civilians. (Reporting by Andrew Heavens; Editing by Louise Ireland)