Dismissed by the Islamist-dominated regime in Khartoum as "Ethiopian aggression", the rebel attacks constitute the first major combined operation by northern and southern opponents of President Omar el-Bashir.
Among those being called to arms are students at Khartoum University which has been closed to allow enlistment in paramilitary units of the Popular Defence Force (PDF). Sudan has an active military force of about 89,000 as well as about 15,000 PDF members.
"Scores of mujahedin [holy fighters] are now hurrying towards the theatre of operation in order to defend the eastern front and teach the aggressors and the traitors a lesson they will never forget," Sudanese radio quoted General Bashir as saying.
The offensive along the Ethiopian border was launched by the combined forces of the southern-based Sudan People's Liberation Army (SPLA) and the National Democratic Alliance (NDA), which has its headquarters in Eritrea. Two of the towns which fell, Kurmuk and Qasan, lie about 350 miles south-east of the capital. In addition to attacking army garrisons, the insurgents have also advanced to within striking distance of the Damazin hydroelectric plant which provides 80 per cent of Khartoum's power.
The SPLA has been fighting since 1983 to resist the imposition of Islam by the Arab north on the Christian and animist south. Sudanese government allegations of Ethiopian involvement in the recent attacks have been at least partially supported by independent reports of artillery shelling of Kurmuk and Qasan from inside Ethiopian territory.
Though once friendly towards Khartoum, Ethiopia's current government shifted allegiances as evidence emerged of Sudan's sponsorship of terrorism in the region.
Ethiopia has denied any part in the attacks, saying the capture of the towns was the work of the SPLA alone.
The Khartoum regime has said Sudan will help 1.3 million Ethiopian and Eritrean refugees in the country take up arms against Sudan's easternmost neighbours. Khartoum stands accused by Ethiopia, Eritrea and Uganda of using force to export its brand of Islamic fundamentalism abroad.
The real question is whether the fighting marks a turning-point in the wavering fortunes of the opposition forces or just one more false alarm. The NDA was predicting that Sudan's Islamic fundamentalist government would fall last summer after an uprising across northern Sudan. But such an uprising failed to materialise.
However, the muted response to this week's government-sponsored demonstrations in Khartoum indicates that the populace is tiring of the empty promises and continued war-mongering of its rulers.