Ban "welcomes the signing of a memorandum of understanding on non-aggression and cooperation between the governments of Sudan and South Sudan in Addis Ababa on 10 February 2012," a UN statement said.

The UN chief "urges both governments to maintain the positive spirit that led to the signature... and abide by its provisions."

The neighbouring nations were in African Union-led negotiations Friday in Addis Ababa. According to the pact, the two sides agreed to have "respect for each other's sovereignty and territorial integrity" and to "refrain from launching any attack, including bombardment."

However, sources said Sudan's military has stopped the return of South Sudanese by barge on the White Nile river because of suspicions they are also being used by the South to reinforce troops near the tense border.

"No further barges carrying passengers can go from Kosti," one source told AFP, asking not to be identified.

Border tensions have mounted since South Sudan split from Sudan in July, becoming the world's newest nation.

Negotiations between the two former civil war foes have been marred by eruptions of violence along the border, including in the contested Abyei and Blue Nile states.

South Sudan took three quarters of Sudan's oil when it gained independence, but all pipeline and export facilities are controlled by the north. Last month, the South halted oil production after Juba accused Khartoum of stealing $815 million worth of crude oil.

The International Organisation for Migration (IOM), an intergovernmental body, last month said it had received international funding to transport 7 000 people from Kosti, about 200 kilometres south of Khartoum, on the two-week barge journey to the South Sudanese capital of Juba.

They are among up to 700 000 ethnic southerners estimated to still be in Sudan ahead of an April deadline for them to either go South or normalise their status with the Khartoum authorities.

By late last year the IOM had already transported by barge more than 17 000 people considered particularly in need and who had been stranded at Kosti for months in rough conditions awaiting their journey south.

Sources said the military did not give a reason last week for stopping the next round of barge transport, but it apparently suspects the vessels are transporting soldiers and their equipment towards the frontier after dropping off the southerners.

In a statement to AFP, the IOM said only that it is "looking into this and to alternatives" for transporting the returnees.

Since last year Sudanese troops have been fighting in Blue Nile and South Kordofan states, along the poorly defined border, against ethnic minority insurgents who fought beside the former rebels now ruling South Sudan.

The South gained independence from Khartoum last July after decades of civil war.

Each side has accused the other of supporting rebels inside their respective borders, and other unresolved issues have sparked fears of renewed fighting between north and south.

Since late last year the IOM has also returned some southerners by train. The agency said in January that it had funding for another six trains to transport about 8 600 people.

The UN's High Commissioner for Refugees, Antonio Guterres, has said he believed most of the 700 000 in Sudan want to go South.

On Sunday, Khartoum and Juba agreed to cooperate in the transfer of ethnic southerners to the new nation, the official SUNA news agency said.

Khartoum's Social Welfare Minister, Amira al-Fadel Mohamed, signed a memorandum of understanding on the issue with South Sudan's Minister of Humanitarian Affairs Joseph Lual Achwel, SUNA said.

The agreement covers road, air and river transport, it added.

Despite the deal, the IOM and the UN have said it is logistically impossible for all southerners in Sudan to either move south or obtain official status in Sudan before the April 8 deadline.