The armies of South Sudan and Sudan on Monday said they were pulling troops from contested border areas, in the latest attempt to set up a buffer zone after fighting last year.

Defence ministers from Juba and Khartoum agreed last Friday on steps to implement the demilitarised zone, which was never put into effect despite commitments by their presidents last September.

A regional political expert expressed doubt that the latest effort will succeed, calling the deal poorly drafted and difficult to monitor.

South Sudan's army spokesman, Philip Aguer, said soldiers would take around two weeks to withdraw southwards from a series of flashpoint border areas.

Troops must "start moving to the designated areas, 10 kilometres (six miles) away from the buffer zone," Aguer told reporters, reading a letter with orders from the army chief of staff.

In Sudan, a statement on Monday from Defence Minister Abdelrahim Mohammed Hussein said his forces were committed to the timetable signed under African Union mediation last Friday in Addis Ababa, Ethiopia.

"From yesterday our troops started withdrawing from the buffer zone," he said.

President Omar al-Bashir and his South Sudanese counterpart Salva Kiir committed in September to "immediately" implement the demilitarised area as part of key agreements they hailed as ending conflict, after battles along their undemarcated frontier in March and April.

The September pacts also called for an opening of the frontier for trade and passage, and a resumption of South Sudanese oil exports through northern pipelines.

None of those measures took effect because of Khartoum's accusations that South Sudan supports rebels north of the border, particularly in South Kordofan and Blue Nile states.

Juba denies the charge and in turn says Khartoum backs insurgents on its soil.

"It's very difficult to monitor, to verify," a regional political expert said of the demilitarised zone, which is supposed to be supervised by observers from Sudan, South Sudan and abroad.

The expert, who asked not to be named, said the pact is vague on some points including who would provide a battalion of troops to protect the monitors.

"This agreement is very bad," he said, adding that some people in both Sudan and South Sudan have no interest in a troop pullout.

"This process will stop, for sure," the expert said. "This current position between peace and war is suitable for them."

Troops are to be withdrawn to their respective sides of the buffer by March 24.

As Sudan and South Sudan announced their moves to demilitarise the border, Khartoum's forces said they clashed with rebels of the Sudan People's Liberation Movement-North (SPLM-N) in Blue Nile.

There were casualties on both sides when government forces "defeated" rebels who attacked a military post on a main north-south road at Surkum, northwest of El Kurmuk town Sunday and Monday, army spokesman Sawarmi Khaled Saad told AFP.

On Sunday the SPLM-N said the rebels had "tactically withdrawn" after an attack which left them in control of Surkum for several hours.

SPLM-N fighters were allies of southern rebels during the civil war that led to South Sudan's independence two years ago under a 2005 peace agreement.

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