Sudanese warplanes bombed South Sudan's oil-rich border regions overnight, wounding several, as violence persisted despite international calls for restraint, officials said Tuesday.

Bomber aircraft hit Panakwach and Lalop in the South's Unity state, as well as on the border post of Teshwin, a contested zone that has seen heavy fighting in recent days, state governor Taban Deng said.

The air strikes, some reaching around 25 kilometres (15 miles) from the frontline, followed bombing raids on the state capital Bentiu earlier on Monday, and continued until the ‘early hours’ on Tuesday, Deng said.

"There are wounded people who have been evacuated to Bentiu hospital, some of them are farmers, some are soldiers," Deng told reporters in Bentiu.

After heavy fighting that broke out last month, both Sudan and South Sudan are reportedly reinforcing troop numbers and digging into trenches along their contested border.

Deng said that at present "with the exception of aerial bombardment, the front line is quiet."

However, Mac Paul, the South's deputy director of military intelligence, warned he had "information from our sources the Sudanese army is mobilising for a push on Bentiu," claims that could not be confirmed.

Bentiu lies at least 60 kilometres (40 kilometres) from the frontline with Sudan's army, and large numbers of Southern troops and tanks have moved into the border zone to bolster defences.

South Sudan said on Sunday it had completed a withdrawal of its forces from the disputed oil town of Heglig – seized from Khartoum's army on April 10 – but has warned it will fight back if Sudan does not end its aerial attacks.

"We cannot be sitting at the mercy of the Sudanese air force," Deng added. "If nothing is being done, for sure we will retaliate... we are very serious about this. We are not joking."

Sudanese President Omar al-Bashir, making a visit to Heglig on Monday, said the time for talks was over with the South Sudan government, which he has previously described as an ‘insect’ that must be eliminated.

Khartoum has repeatedly denied it has launched air strikes on the South, but United Nations chief Ban Ki-moon on Monday deplored the cross-border air raids, and called on both nations to prevent the fighting escalating further.

Southerners say they are increasingly angry at what they see as international inaction on Khartoum, after they complied with the demands to withdraw.

"We were asked to withdraw from Heglig: we did. They (Sudan) have been asked to stop the aerial bombardment and incursions into South Sudan, they have not done so," Deng said.

"We are capable of defending ourselves, including going back to Heglig... I think you people should take us seriously on this. The underdog can also bite."

Sudan however accuses the South of supporting anti-government rebels from its conflict-hit western region of Darfur as well as those fighting in Blue Nile and South Kordofan states.

The South, which split from Sudan in July 2011 following an independence referendum, denies backing opposition movements in the north, but in turn accuses Khartoum of supporting rebels in its own territory.

The recent violence is the worst since South Sudan won independence after a 1983-2005 civil war in which an estimated two million people died.