KHARTOUM: Millions of migrating cows are at risk of death on the Sudan-South Sudan border, nomads warned on Thursday, because the two nations have failed to open the frontier as agreed several months ago.

The cattle have fallen victim to the inability of Sudan and South Sudan to implement key security and economic agreements which their presidents signed in September.

The leaders had hailed the pacts, which called for a reopening of border points, following a March-April war on the undemarcated frontier.

"We want to cross the river with our animals but South Sudan has closed the area," Mahmoud Gibril, a local leader from the nomadic Rezeigat tribe, told AFP.

He said two million cows, their herders and families have been stuck north of the Bahr al-Arab River since mid-December, waiting to go south on their annual dry-season migration.

"These cows are suffering from a lack of grass and water," Gibril said.

Later in December, the Rezeigat and "armed groups" from South Sudan clashed around the river at Samaha, according to the Sudanese military.

The area, which Sudan considers part of East Darfur state, is disputed by the two countries.

Gibril said South Sudanese security forces have a compound along the waterway "and they don't allow anyone to come near."

The blockade has concentrated the nomads and their animals in a small area, where there is no Sudanese military or police presence, he added.

"If this continues until the beginning of April, most of these animals will die," Gibril said.

A South Sudanese official had no details but said "definitely these kind of things can happen" as long as the September deals have not been implemented. "Hostility on any part of the border affects the whole border," he added.

The frontiers have never been as tightly shut as this year, a foreign diplomat told AFP.

"I am worried at the moment about developments on the ground," the diplomat, said, asking for anonymity.

Rough estimates show that about 7.5 million animals, mostly cattle, have been prevented from reaching their traditional water points and grazing areas, the United Nations Food and Agriculture Organisation said.

"Already last year, there had been problems of livestock congestion at the border. Implementing the September agreement could have prevented similar problems this year," the organisation said in a statement to AFP.

Four tribes are reportedly affected in South and East Darfur, in South Kordofan state and the Abyei region, the FAO said, but there are "divergent reports" about who is preventing the migration through areas that are home to various armed groups.

Another Rezeigat cattle herder, Idam Abubakr, told AFP that the nomads depend on water from the Bahr al-Arab but South Sudan had "occupied" Samaha which prevented passage.

"Two weeks ago, 90 cows went to the river and were taken by South Sudan's army," Abubakr said.

FAO said the current over-concentration of livestock will eventually lead to rapid depletion of pasture and water points, which for the moment are still plentiful.

"If no solution is found until March or April, the condition of the animals is likely to deteriorate rapidly," the FAO said.

The September agreements, along with opening the boundaries for trade and passage, called for a demilitarised buffer zone along the frontier and a resumption of South Sudanese oil exports through northern pipelines.

Khartoum's accusation that South Sudan, which separated in 2011, supports rebels north of the border has been a major obstacle to implementing the deals.

Gibril said the animals may not have the patience of politicians.

"The cows, sometimes their owners cannot stop them. They can cross the river without permission from anyone and this will create real trouble for their owners," he said.