Foreign aid workers took shelter in a hotel in this southwestern Pakistani city after authorities warned them that Taliban (search) rebels were planning suicide attacks against their offices, officials said Sunday.

Some 25 expatriates from the United Nations and Western aid groups have left their residences and moved to Quetta's Serena Hotel — days after suspected Taliban gunmen killed five aid workers in neighboring Afghanistan.

A Pakistani government agency responsible for security at refugee camps in southwestern Baluchistan province alerted the U.N. refugee agency and five other non-governmental organizations on Saturday to the suspected threat that targeted aid groups with American and British employees.

A letter sent by the Pakistani agency to the aid groups and obtained by The Associated Press on Saturday identified the lead planner as a formerly unknown Taliban member, Mullah Hashim Sagzai (search), who is believed to live in a refugee camp in Baluchistan, a southwestern province bordering Afghanistan.

It said Sagzai's operatives were seeking access to the aid groups' premises to stage a suicide bombing.

The head of the U.N. High Commissioner for Refugees (search) office in Quetta said they were taking the warning seriously and it had forced U.N. agencies and the Western aid groups to suspend operations in Baluchistan.

"Our head office directed us to leave the residential premises and stay at the Serena Hotel," Yousaf Adam, who is a Sudanese national, told AP. "Of course this is a threat to our lives."

About a dozen police were outside the hotel Sunday, checking cars entering for weapons. The expatriates there include British, French and Sudanese nationals, an intelligence official said on condition of anonymity.

On Sunday, a small homemade bomb exploded in a field in a western residential area of Quetta, about 2 1/2 miles from the hotel. Police official Abdul Jalil said no one was injured, and the bomb had not targeted Westerners.

The aid groups warned of the Taliban plot were UNHCR, the U.S.-based Mercy Corps International, British-based Global Partner, Ireland's Concern, the French Tear Fund, and the Association of Medical Doctors of Asia.

They provide tens of thousands of Afghan refugees in Baluchistan with food, shelter, medical aid, water and sanitation equipment and long-term assistance like building schools.

Remnants of the Taliban and their al-Qaida allies are believed to be hiding in Baluchistan and other border areas of Pakistan since being driven from Afghanistan in the U.S.-led military campaign after Sept. 11.

Taliban insurgents are suspected of attacking aid workers in Afghanistan.

On Wednesday, three foreign and two Afghan workers for the Nobel Peace Prize-winning Medecins Sans Frontieres (search) — Doctors without Borders — were shot and killed in northwestern Afghanistan, an area that had generally been safe for relief agencies. A purported Taliban spokesman claimed responsibility.

On Sunday, a convoy of U.N. election workers in southeastern Afghanistan was ambushed on a road but managed to escape unharmed after seeking help from U.S.-led coalition forces.