Fighting involving South Sudanese forces near Sudan's Heglig oilfield has not affected the field's 60,000 barrels per day output, its Chinese-led operator said on Wednesday.

Clashes broke out on Monday in several areas along the border between Sudan and South Sudan, which became independent in July last year, with each side blaming the other.

Sudan said on Tuesday it had expelled southern army units from the area around the Heglig field, which is on the Sudan side of the border and is one of the largest oil fields remaining in the country.

"The output is unchanged.....at 60,000 barrels a day," Abdallah Suleiman, Vice President of Greater Nile Petroleum Operating Co. (GNPOC), told reporters during a visit organised by the government to the Heglig field.

"The production facilities were not affected by the events. It's normal business," he said, standing near worker housing facilities that were destroyed by the fighting.

The consortium, owned by Chinese, Malaysian, Indian and Sudanese companies, would go ahead with plans to increase output to 70,000 bpd, he said.

"We will reach this in two months," Suleiman said.

The Heglig field is key to the Sudanese economy because it contributes almost half of the country's output of 115,000 bpd.

Sudan lost three quarters of its output when South Sudan became independent in July last year. Both countries are locked in a row over how much the landlocked new nation should pay to export its crude through the north.

Suleiman also said an export pipeline from South Sudan crossing the border was not damaged during the latest fighting.

South Sudan said Sudan launched airstrikes on major oilfields in its Unity state on Tuesday and GNPOC, which also has facilities there, confirmed they had been hit.

Sudan denied launching air strikes but said its ground forces had attacked southern artillery positions which had fired at the disputed Heglig area.

In January, South Sudan shut down its entire output of 350,000 bpd to stop Khartoum taking some oil for what it calls unpaid transit fees.