The U.N. mission in East Timor has withdrawn its staff from a third Timorese town after another violent militia attack on a U.N.-escorted aid convoy, the United Nations announced today.

The attack Sunday marked the third such incident in the last week, raising questions about the viability of the U.N. operation and the Indonesian government's guarantees of security for diplomats in the troubled territory, which has been occupied by Indonesia since 1976.

At least three people were injured in the attack, which occurred on the eastern edge of Liquica, a town about 28 miles west of the capital, Dili. Five people are still missing and a Foreign Ministry official said they could have fled or might be in hiding.

The attack occurred when pro-Indonesian militiamen armed with machetes and homemade rifles attacked a humanitarian aid convoy taking relief supplies to displaced people in Liquica. The militiamen later attacked a U.N. helicopter that flew in to evacuate the wounded.

Diplomats described the incident as the most serious yet in a string of violent attacks by the military-backed militias, who are trying to disrupt a vote planned for August that could bring independence to the territory. This incident, they said, suggested that recent attacks are part of a pattern aimed at sabotaging the U.N. operation and forcing a delay or cancellation of the vote.

"There is a certain pattern of incidents, of threats, from militias to U.N. personnel, and that is a major concern to us," Ian Martin, the head of the U.N. mission, said in a statement. He said the failure of Indonesian police to provide protection for the aid convoy, despite repeated requests, shows "an inexcusable lack of action."

Diplomats said the latest incident raised the stakes in the militia groups' apparent plan to intimidate U.N. staff and prevent a free vote on independence. "This is extremely serious," said a Western diplomat in Jakarta who follows developments in Timor. "This is a crucial test for the U.N. and for all of us."

Dino Djalal, the Foreign Ministry official assigned to Timor to prepare for the referendum, said in a telephone interview, "Any attack on the U.N., or U.N. vehicles, is of concern to the government. They must be assured of a safe and secure environment."

He said the government is still investigating various versions of what happened during the attack, because, he said, "there's a lot of stories we need to cross-check."