(Writes through)

KHARTOUM, March 17 (Reuters) - Sudan threatened on Thursday to close its recently reopened border with South Sudan, accusing its neighbour of supporting rebel groups.

Stoking tensions further, Sudan's cabinet said it would tighten restrictions on South Sudanese people living in its territory and prosecute any without proper papers, state media reported.

There was no immediate explanation of what prompted the announcements - though they came a day before Sudanese government officials were due to meet rebels who have been fighting along the long frontier.

"If the South Sudan government does not refrain from supporting the rebels, we will be forced to close the border with the south once again," presidential aide Ibrahim Mahmoud said.

Sudan regularly accuses its neighbour of backing insurgents in its Darfur, Blue Nile and South Kordofan regions.

South Sudan, which split away from Sudan in 2011 after decades of civil war fuelled by ethnicity and oil, dismisses the allegations and accuses Khartoum of arming militias in its territory.

The position of the border, particularly in the contested Abyei area, has been a constant source of tension, as has the legal status of southerners left living in the north and vice versa.

The cabinet announcement said southerners in the north would be treated as foreigners when it came to education and healthcare.

The border, regularly crossed by traders and pastoralists, was closed after the 2011 split and only re-opened in January.

Sudan's opposition Umma party is also expected to attend Friday's talks in neighbouring Ethiopia. (Reporting by Khalid Abdelaziz; Writing by Eric Knecht; Editing by Andrew Heavens)