A confrontation in the fertile region could restart the civil war between north and south that raged for more than two decades and ended only in 2005, said Princeton Lyman.
The newly appointed special envoy described the situation in Abyei as "very tense." By bringing in military hardware, both parties have violated the terms of a recently reached security deal stating that forces would be withdrawn.
"That is very worrisome," Lyman said in an interview in the Ethiopian capital of Addis Ababa.
"The danger is that a confrontation in a place like Abyei could get out of hand," he said. "And that could lead them to wholesale war with very serious consequences."
Southern Sudan is due to become independent in July after voting for secession in a peaceful referendum in January, part of a peace deal that ended one of Africa's longest-running conflicts. Abyei was also promised a referendum but it was not held after northern and southern leaders disagreed on who was eligible to vote.
Many outstanding issues remain to be settled between the two sides and Lyman said Abyei is at the top of the agenda.Continued