Updated

Two Sudanese workers for aid group World Vision were in critical condition on Friday, a humanitarian source said, following a grenade strike which killed their co-worker during fighting in the Darfur region.

The attack happened on Thursday during a battle which officials in South Darfur state blamed on "differences" between members of the security forces.

Three World Vision employees were wounded.

"Two critical," the humanitarian source said in brief comments.

He asked to remain anonymous but added that the third staffer has been released from hospital.

All of the victims, including the one killed, worked for World Vision, the source confirmed.

The UN's humanitarian agency OCHA said late Thursday that a Sudanese staff member working for an international NGO was killed and three others injured "as a result of a rocket-propelled grenade hitting an NGO office".

It did not name the non-governmental organisation but said the NGO office apparently became caught in crossfire.

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

Rebels have been fighting the government there 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.