KHARTOUM (AFP) - A Sudanese worker for aid group World Vision has died of injuries suffered in a suspected grenade blast during urban fighting in Darfur last week, humanitarian sources said on Sunday as fresh battles erupted.

A second World Vision staff member was killed during the clashes in Nyala city on Thursday, which officials in South Darfur state blamed on "differences" between members of the security forces.

"He suffered head injuries in Thursday's attack and succumbed to his injuries yesterday (Saturday) noon," said one humanitarian source, asking for anonymity.

Mr Ali Al-Za'tari, chief of the United Nations in Sudan, confirmed in a statement that "two Sudanese staff members from the humanitarian non-governmental organisation, World Vision International" were killed.

"A third World Vision staff member was also critically injured," Mr Za'tari said.

The fighting in Nyala was sparked by the killing of a notorious local bandit who was also an officer in the paramilitary Central Reserve Police.

A local resident said fighting resumed on Sunday afternoon with more explosions and shooting heard. Earlier battles had left the city's best hotel damaged, he added.

The World Vision office was caught up in crossfire between the combatants.

A suspected rocket-propelled grenade hit the top of the World Vision building, came down "and exploded on the ground", the humanitarian source said.

The latest incident adds to what the United Nations says is a worsening security situation in Sudan's vast western region.

Rebels in Darfur have been fighting the government for 10 years, but instability has been complicated by inter-tribal fighting, kidnappings, carjackings and other crimes, many suspected to be the work of government-linked militia and paramilitary groups.

The "shoot-out in an urban area of Darfur's most populated city" highlights the region's unstable security environment, Mr Za'tari said.

"If humanitarian work in Darfur is forced to scale back because of the unsafe and insecure conditions for our staff and partners, then many more people in Darfur will suffer," he added.

In February, a UN panel of experts reported "some incidents in which former members of government militias have forcibly expressed their discontent with the current government, especially against the backdrop of rising inflation and unemployment".

The United States charge d'affaires to Sudan, Mr Joseph Stafford, later said Washington was worried "about the deteriorating security situation in Darfur and the conflict between the government forces and the militia".

The UN says an estimated 300,000 people have been forced to flee their homes because of fighting in Darfur this year, more than in the past two years combined.

There were already 1.4 million people in camps for Darfuris displaced by the decade-long conflict.