African nations have started an investigation into allegations by Sudan and South Sudan that they are supporting rebels operating in each other's territory, the African Union said.
The AU and east African bloc, the Inter-Governmental Authority on Development (IGAD), on Monday launched the investigative panel in the Ethiopian capital Addis Ababa.
The three senior military officers begin their six-week mission ahead of an August 7 deadline from Khartoum to shut a pipeline carrying South Sudanese oil for export.
In a surprise move in early June, Sudan gave companies 60 days to stop transporting oil from South Sudan after President Omar al-Bashir accused the Juba government of backing rebels in the north.
Juba denies supporting the fighters and in turn says Khartoum assists rebels on southern soil.
Also on Monday, regional nations began determining the centreline of a demilitarised buffer zone that is to straddle the 2,000-kilometre undemarcated border between the two countries.
"The launch of these mechanisms underscores the seriousness with which the African Union and IGAD regard relations between Sudan and South Sudan," the AU said.
"Since 2010, Africa has been working tirelessly to promote two mutually viable states, and these current allegations threaten this objective, and in fact pose a threat to regional peace and security."
Although the buffer zone was being set up, analysts say Bashir ordered the pipeline shut after continued rebel attacks humiliated the Sudanese authorities.
"Everybody knows that both of them are supporting rebels," an African diplomat told AFP last week. "It's clear as daylight."
At the same time, Juba continues to provide "logistical and coordination support", including access to rear bases in the South's Unity state, for the Sudan Revolutionary Front (SRF), Small Arms Survey said.