UN Secretary-General Kofi Annan voiced deep concern on Tuesday over the recent escalation of fighting along Chad's eastern border with the troubled Darfur region, western Sudan, and its spillover to the Central African Republic.

"He observes that the increased violence is heightening political tensions in Chad and that its spillover effect is undermining international efforts to contribute to the stabilization of the situations in neighboring Darfur and the Central African Republic," a UN spokesman told reporters.

"He strongly condemns the attacks against refugee camps in southern Chad and stresses the need to respect the safety and dignity of refugees and internally displaced persons," the spokesman said.

Annan also firmly condemned the killing in northern Central African Republic of two doctors on a mission supported by the UN system to deliver vital medical assistance to thousands of civilians trapped by the worsening violence there, he noted.

The UN leader called on the authorities of the Central African Republic for swift action to bring those responsible for the killings to justice.