It is a sight the South Sudanese hoped had gone for good: twisted metal shards embedded in thigh-deep bomb craters left by an airstrike blamed on former foes in neighbouring Sudan.

Officials from Juba and Khartoum are due to resume on Tuesday African Union-mediated talks to resolve a furious row over oil transit fees, which many fear risks tipping the bitter rivals back into bloody war.

The jet strikes last week at El Nar, an oilfield in South Sudan's Unity state, are a grim reminder of what is at risk if talks in the Ethiopian capital Addis Ababa fail again, Southerners and experts warn.

"It is sending a signal before the talks," said Mohamed Lino Benjamin, director general of the South's petroleum and mining ministry, standing at the lip of the craters, some 20 kilometres (12 miles) from the border with Sudan.

Sudan denied any involvement with the bombing and lodged its fourth complaint with the UN Security Council, accusing the breakaway fledgling nation of funding rebels in two border states and carrying out joint attacks on contested territory.

South Sudan split from Khartoum last July after decades of civil war, taking with it three-quarters of the oil -- but all pipeline and export facilities are controlled by Sudan.

It took the drastic decision to halt production in January, despite oil making up 98 percent of its revenue, after Sudan started seizing the crude in lieu of a deal on transit fees.

"With high rhetoric, continued cross-border tensions restrict the space necessary to advance the talks in a meaningful way," International Crisis Group analyst Zach Vertin told AFP.

"Both sides need a deal on the export of oil, but every provocation hardens attitudes in an environment where trust is already thin."

The crisis between the two nations has become a major threat to regional peace and security, United Nations chief Ban Ki-moon warned in January.

At El Nar, people climbed in and out of the craters, picking up metal fragments as though searching for a reason why this vast expanse of cracked black earth was targeted.

"The planes were very fast and kept turning on their sides... two bombs fell and killed a number of cows," said village chief Miakol Lual, pointing northeast towards Sudan when asked where they came from.

Chom Juaj, Vice President of the Greater Nile Petroleum Operating Company (GNPOC) that manages these fields, thinks it was a warning message from Sudan, keen to flex its muscles over the South's oil fields but not show "its full weight".

Juaj thinks Khartoum bombed the fields out of "frustration" over the shutdown that dealt its oil-reliant economy a second blow -- after losing 350,000 barrels a day production when the South split away last year.

The strikes, the latest in a string of attacks, missed a large processing facility about a kilometre (mile) away, but Juaj doubts a stronger message will follow if Sudan is "wise."

It has a vested interest in Southern facilities and strong links with major investor China, which depended on the South for some five percent of its oil, while attacks could even trigger explosions in Sudan's connected facilities.

"It sounds like it's more of a message than a deliberate attempt, but it will just drive them one step closer to a war in which both sides will lose massively," said an international observer close to the talks.

South Sudan already saw itself as a victim and the attack was "like prodding a hornet's nest," while Sudan is already fighting rebels on three fronts, and could not afford another war, he said.

Requesting anonymity, he said that diplomats have long mooted what would happen in this scenario.

"If the north attacked the oil fields, which is an option they have as they have superior air power, that's a signal that we've ratcheted up tensions," the observer added.

"On a scale of 1-10, where 10 is war... we've reached about seven or eight," he said.

The South's oil is key to paying its bloated ex-rebel army, which continues to absorb militia forces as part of peace deals to stem multiple rebellions in the new nation.

But Juba appears confident of surviving without oil revenues, which although key for government salaries have far less impact on the majority rural population, with many basic services already funded by donors.

Khartoum -- itself beset by a weakening economy -- has also been angered by Juba's efforts to ensure its "economic independence," including recent agreements with Kenya and Ethiopia for alternative oil pipelines.

"The game changed when (South Sudan President) Salva Kiir said 'I'm not going to continue with interdependence' -- which is what the international community wanted," the international observer said.