KHARTOUM, May 19 -- Sudan has rejected accusations that it supported rebels in South Sudan.

"Khartoum is committed to the good neighbourliness relationship with South Sudan and does not intervene in its internal affairs," Foreign Ministry spokesman Ali Al-Saddiq told reporters late Monday.

He was responding to accusations by South Sudanese officials that Khartoum provided support to rebels seeking to seize Malakal, the capital of South Sudan's oil-rich Upper Nile State.

"The Sudanese government has spared no effort to achieve stability in South Sudan," Al-Saddiq said, adding that worsened instability there also hurts Sudan.

"The internal situation in South Sudan has affected the implementation of cooperation agreements between the two countries," Al-Saddiq said.

Earlier on Monday, the government of South Sudan accused Khartoum of backing rebels in South Sudan, who have been launching a large-scale attack against the strategic town of Malakal since Friday.

"We have evidences proving Khartoum's support for the rebels," South Sudanese Information Minister Michael Makue said over phone from Juba.

South Sudan, which became independent in 2011, plunged into violence in December 2013, when fighting erupted between troops loyal to President Salva Kiir and defectors headed by his former deputy Riek Machar.

The conflict soon turned into an all-out war, with violence taking on an ethnic dimension that pitted the president's Dinka tribe against Machar's Nuer ethnic group.

The clashes have left thousands of South Sudanese dead and forced around 1.9 million people to flee their homes.