KHARTOUM (Reuters) - Clashes erupted between Sudan's army and rebels in the oil-producing state of South Kordofan, both sides said on Monday, a week after the insurgents shelled the state capital near the border with South Sudan.

The fighting, which has been rumbling since June last year, has forced more than half a million people to flee and stoked tensions between Sudan and South Sudan, former enemies in a civil war that was fueled by oil, ethnicity and religion.

Khartoum accuses South Sudan of backing the rebel Sudan People's Liberation Movement-North (SPLM-North), charges dismissed by the South's government.

Sudanese army spokesman Al-Sawarmi Khalid said the army had killed 15 rebels near South Kordofan's main city Kadugli on Sunday. "Many rebels were also wounded," he said.

The army also repelled an SLPM-North attack near the villages of Annagarko and Hgerjawad villages on Friday, he added.

SPLM-North confirmed fighting took place near Annagarko and Hgerjawad and said 20 soldiers were wounded.

The rebel force also said it clashed with the army further along the border in Surkum in Blue Nile state on Monday, but the army denied the report.

The armies of Sudan and South Sudan have also clashed across the border since the South declared independence in July last year, under the terms of the 2005 peace deal that ended their civil war.

The two countries agreed to set up a buffer zone along the border last month after coming under international pressure to end the violence.

But there has been scant progress in parallel indirect talks between Khartoum and SPLM-North, which fought as part of the southern rebel army during the civil war.

SPLM-North, which accuses the government of marginalizing large parts of South Kordofan and other border areas, has formed an alliance with other rebel groups to try and topple the country's veteran President Omar Hassan al-Bashir.

(Reporting by Khalid Abdelaziz; Writing by Ulf Laessing; Editing by Andrew Heavens)