Darfur rebels wounded in the latest fighting with Sudanese troops have gone to South Sudan for treatment, the army said on Tuesday, as Khartoum pushes Juba to end alleged backing for rebels.

The army and insurgents gave conflicting accounts of Monday's fighting, which came while Sudanese negotiators at fragile peace talks in Addis Ababa turned down South Sudan's proposal for settling oil fees and other critical issues by a United Nations-imposed deadline of August 2.

Khartoum said security is a key priority and issues such as South Sudan's "support" for rebels need to be settled.

In a statement carried by the official SUNA news agency, Sudanese army spokesman Sawarmi Khaled Saad said the government killed more than 50 fighters of the Justice and Equality Movement (JEM) and wounded a large number of others.

The fighting erupted just inside South Kordofan state near southeastern Darfur.

Saad said a "big number of vehicles were seen carrying the injured elements of the rebels for treatment in South Sudan."

Casualty claims are difficult to verify in the region, where access is restricted.

JEM on Tuesday denied that any of its fighters had been killed in the battles or that wounded had been moved across the undemarcated border.

Rebel spokesman Gibril Adam Bilal said his forces had control of the Tabaldi oil field as well as the Karkade and Tabun areas since Monday evening.

Sudan accuses South Sudan of working with the JEM and of backing insurgencies in South Kordofan and Blue Nile states.

The South denies supporting the rebels but suspected JEM fighters were seen alongside its troops during border fighting between Sudan and South Sudan in April.

The JEM denies any presence in South Sudan, which accuses the north of backing insurgents in the South as well.

The UN has called on both sides to halt any such support, under a May 2 Security Council resolution which ordered a ceasefire along the border.

The resolution gave the two sides until Thursday of next week to settle critical issues, including a dispute over oil, unresolved after the South's separation in July last year.

At African Union-led talks in the Ethiopian capital, Sudan on Monday rejected South Sudan's proposal of a higher oil transit fee and an $8.2 billion financial deal.

"We think security is a prerequisite," Mutrif Siddiq, a member of Khartoum's delegation, told reporters.

South Sudan separated with about 75 percent of Sudanese oil production.

But the export infrastructure remained in the north and the two sides' failure to agree over how much the South should pay to send its crude through northern pipelines has been at the heart of tensions between the two countries.

In January, Juba cut off all oil production.

Pagan Amum, South Sudan's chief negotiator, said on Monday that his government was ready to resume oil exports if "reasonable" transport fees are agreed.

He outlined a proposal whereby Juba would pay up to $9.10 a barrel to move its oil through Sudan.

Khartoum earlier demanded as much as $36 per barrel, which includes tariffs and transit, processing and port fees.

South Sudan said that "in the interest of peace" it was offering Sudan a multi-billion-dollar financial package over three years, including a cash payment and debt forgiveness to help fill the massive fiscal gap Sudan reported after it lost its main source of hard currency when the South separated.

Dismissing the offer, Siddiq ruled out any comprehensive deal by the August 2 deadline but said he remained hopeful in the longer term.

In Monday's fighting, army spokesman Saad said government troops repulsed the JEM at Karkade and another area, Um-Shuwaika, destroying 25 of its vehicles.

He made no mention of fighting around an oil field but said the army lost "a number of martyrs" in the action with rebels whose goals were dictated "by foreign circles."

JEM claimed it killed "tens" of government troops.

On Saturday, South Sudan accused Khartoum of a new cross-border air raid and said in response it would negotiate only through AU mediators, not face-to-face.

Sudan said it retaliated inside its own territory to an attempted JEM attack which Siddiq called a "stab in the back" by Juba.