South Sudan said Tuesday Sudanese war planes have launched air strikes against its territory, the latest in a series of attacks that threaten to scupper international efforts to restart peace talks.

"South Sudan views the current aerial bombardment... as a serious threat to both regional and international peace and security," South Sudan's Information Minister Barnaba Marial Benjamin told reporters.

The former civil war foes fought heavily in contested border regions last month, the worst fighting since the South won independence last July and sparking international concerns of a return to all-out war.

The bombings Monday and Tuesday targeted the Werguet area of Northern Bahr el Ghazal state, a border region close to Sudan's war-torn South Darfur state, Benjamin said.

"South Sudan is watching this crisis very closely... we will be forced also to react to these acts of aggression," he added, without giving further details.

The air strikes could not be independently confirmed, and Sudan's army repeatedly denied Southern claims of air strikes during weeks of bitter border conflict. The last air strike reported by the South was on May 9.

Both sides say they are committed to peace but missed a United Nations Security Council demand that they resume the talks by last Wednesday. The South has said it is ready to talk and accused Khartoum of stalling.

The African Union is working hard to resume talks between the foes, with its chief mediator, former South African president Thabo Mbeki, embarking on rounds of shuttle diplomacy between the two capitals.

Mbeki during a visit to Juba on Monday said he was hopeful the two sides could set a date to restart talks by the end of the week. Mbeki was expected to meet Sudanese President Omar al-Bashir late Tuesday.

However, Benjamin demanded the UN and AU order Khartoum to end its air raids.

"The Sudan government has been dragging its feet trying to set an agenda (at the talks)," Benjamin added. "What do we get with the presence of president Mbeki in Juba? The Sudan government then attacks."

Benjamin also claimed that Sudanese planes flew over the South's capital Juba on Monday.

"There were flights of Antonov planes over Juba that did not respond to our air communication enquiries... a violation of international law," he said.

South Sudan broke away from Sudan in July after a 2005 peace deal ended one of Africa's longest civil wars, which killed about two million people.

But tensions soon flared again over a series of unresolved issues, including the border, the future of disputed territories and oil.

The South separated with about 75 percent of the former united Sudan's oil production, but Juba still depends on the north's pipeline and Red Sea port to export its crude.

A protracted dispute over fees for use of that oil pipeline infrastructure led South Sudan in January to shut its oil production after accusing the north of theft.

The Security Council gave both sides three months to conclude the talks.