Peace talks between Somalia's weak interim government and Islamists are deadlocked in the Sudan capital.
The Union of Islamic Courts wants Kenya's role as mediator to end, while the government insists it stays.
Fighters from the two sides are now less than 30km apart, leading to fears of direct clashes.
Observers further fear a regional conflict, as Ethiopia backs the government and its rival Eritrea is accused of backing the Islamists.
Before arriving in Khartoum, the Islamist delegation said they would not take part in the talks unless Ethiopian troops leave Somalia.
Ethiopia denies having a fighting force in Somalia, but says it has hundreds of military trainers with the government.
In order to break the impasse over who chairs the talks, diplomats have suggested Sudan.
While the UIC objects to Kenya, the government accuses the other co-chair, the Arab League, of bias.
UN envoy to Somalia Francois Fall, however, says he remains optimistic.
"It is too early to say the peace conference is collapsing... There are a lot problems facing the conference which will gradually be resolved."
Read More
