GENEVA (Reuters) - The United Nations refugee agency said on Friday it had reduced its activities and staff movements in volatile eastern Chad due to worsening violence.

Two gendarmes guarding its camp at Touloum, which holds 23,500 Sudanese refugees from Darfur, were killed by armed Chadian men on Wednesday, it said.

Two other gendarmes were severely wounded in the attack, which came a day after hijackings of vehicles from aid agencies.

"As a precautionary measure, we will be carrying out only essential tasks such as follow-up on protection cases, food and water distribution and health assistance," Jennifer Pagonis, spokeswoman of the U.N. High Commissioner for Refugees (UNHCR), told a news briefing.

The UNHCR feared that a rebel attack on the Sudanese capital Khartoum a week ago, in which more than 200 people were killed, "could further destabilise the security situation" in eastern Chad. Sudan blamed Chad for the assault and cut diplomatic relations.

An Antonov plane, of the type usually used by the Sudanese government, last Sunday flew over Oure Cassoni refugee camp near Bahai, according to Pagonis. The camp, just six km (four miles) from the border, holds 28,000 Sudanese refugees.

"It was very scary for the refugees," she said.

About 8,000 of an estimated 13,500 Sudanese refugees who fled military operations by Sudan's army and its Janjaweed militia allies in Darfur in February and March are still camping in the open at Birak, a small Chadian border town.

Their transfer to Mile refugee camp two weeks ago had to be postponed because of the extremely volatile security situation.

There are 250,000 Sudanese refugees scattered in 12 refugee camps in eastern Chad and 180,000 internally displaced Chadians, according to the UNHCR.