Sudan has reportedly declared a state of emergency in areas bordering South Sudan following month-long border clashes and after South Sudan occupied the north's main Heglig oil field for 10 days.

Although South Sudan left the oil field about a week ago, tension still prevails between the two nations.

President Omar al-Bashir declared a state of emergency in the border states of South Kordofan, White Nile and Sennar, the official Suna news agency said. Other parts of the border were already under a state of emergency. According to Suna, the state of emergency "gives the right to the president and anyone with his mandate" to establish special courts, in consultation with the chief justice, reported AFP.

South Sudan authorities have informed the United Nations in a letter that it had planned to withdraw all police from the disputed Abyei region of fertile grazing land, which both the countries had claimed. South Sudan became independent from Sudan last July.

The letter seen by Reuters on Sunday, also said South Sudan was committed to an "immediate cessation of all hostilities" - after the African Union ordered both parties to stop fighting.

"All of these acts of peace are being done to reaffirm and demonstrate with concrete measures my government's true commitment to finding a peaceful solution to the outstanding matters with the Republic of Sudan," said the letter.

As many as 12,000 South Sudanese people who reside in the North have already started gathering in the border town of Kosti in recent weeks after the Khartoum government set an 8 April deadline to either regularise their residency papers or leave the country. Around 350,000 South Sudanese had earlier moved to the North looking for work after a 2002 peace agreement.

They have been become stuck at the river port of Kosti as the Khartoum government has refused to allow barges from the South to take these people as they believe that these barges could be used by South Sudan to carry military equipment or militia northwards. The Khartoum government is demanding guarantees from the South for the same, a BBC report said.

Four people from the UK, Norway, South Africa and South Sudan who were arrested on Saturday by the Sudanese army in the Heglig oil region on charges of illegally entering and spying have been flown to the Sudanese capital Khartoum for further investigation. South Sudanese officials have denied the charges.