Sudan's army is beefing up its bombing capability in the border state of Blue Nile, a US monitoring group said on Friday, after Khartoum was accused of deadly air strikes on a refugee camp in South Sudan.

Satellite imagery has "confirmed" that the military is "rapidly working to enhance air strike and air assault capacity in two air bases recently captured from rebels in Sudans Blue Nile border area," said the Enough Project, which is supported by Hollywood star George Clooney.

The imagery claims to show three helicopter gunships and an Antonov aircraft, frequently used in Sudanese Armed Forces (SAF) bombing campaigns in the past, on the air strip of the state capital Damazin.

It also indicates that the army has built four new helipads in Blue Nile's southern border town of Kurmuk since capturing it from the rebels on November 3.

"The airfield improvements suggest Sudans readiness to widen its aerial bombing campaign in its border areas of South Kordofan and Blue Nile, as well as neighbouring South Sudan," said the Enough Project's co-founder John Prendergast.

Fighting erupted in Blue Nile in early September, between the SAF and the southern aligned rebels, less than two months after the independence of South Sudan, and the former rebel stronghold of Kurmuk was finally captured by the army last week.

On Thursday, southern officials accused the SAF of carrying out deadly air strikes south of the border for the second time this week, including on the Yida refugee camp in Unity state, where 12 people were killed and more than 20 wounded, according to the county commissioner.

Tens of thousands of civilians have fled across Sudan's poorly defined border with the newly independent south to escape the fighting in Blue Nile and nearby South Kordofan, where a similar conflict has raged since June.