South Sudanese internally displaced persons perform a traditional dance as Sudanese President Omar Hassan al-Bashir addresses a crowd during a campaign rally in East Darfur, April 5, 2016. Mohamed Nureldin Abdallah/Reuters

Sudan said on Sunday it would close its border with South Sudan within days if its neighbor did not expel militant groups, the government told state media.

Sudan regularly accuses its neighbor of backing insurgents in the Darfur, Blue Nile and South Kordofan regions that run along its southern border.

South Sudan split away from Sudan in 2011 after decades of civil war fueled by ethnic divides and disputes over oil.

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

Speaking to state media on Sunday, Kamal Ismail, a minister of state in the foreign ministry, said that South Sudan had pledged to expel the militant groups within 21 days during a visit last month.

Khartoum would re-seal the border if its neighbor did not meet this promise, Ismail said.

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.