The fragile security situation in East Timor and a series of attacks on aid convoys is leading to serious doubts as to whether the proposed referendum on the future of the territory can go ahead.

The UN has already delayed the ballot by two weeks, but is now threatening to suspend preparations altogether unless security conditions can be met before the beginning of voter registration in the province in seven-days time.

But whether or not the Indonesians can provide such guarantees is far from obvious.

Under the terms of the New York agreement between the UN, Indonesia and Portugal on East Timor's future, Indonesia must play a neutral role in policing the ballot and protecting UN staff from armed militias opposed to independence from Indonesia.

Indonesia has given repeated assurances that it will protect UN staff, and the army has denied it intends to sabotage the ballot. But in the latest incident on Sunday, one aid worker told the BBC that Indonesian police stood and watched as UN staff were kicked and stoned.

Pro-independence supporters believe the police support the militias

The UN is becoming increasingly frustrated at the lack of acknowledgement from Jakarta as to the seriousness of the problem. Most pro-independence Timorese believe that Indonesian soldiers on the ground are lending active support to the militias.

What is not clear, however, is if the Indonesian military is choosing not to control the militias, or if they simply can't control them.

Whatever the answer to that question, the fragile situation is putting a growing strain on relations between Indonesia and the UN.

Jakarta doesn't want to get the blame for blocking a resolution on East Timor and is highly embarrassed by the attacks on UN personnel. But the Indonesian security forces alone are responsible for rectifying the security situation.

One option that has been mentioned is for the UN to extend its mandate to include an armed protection force. But the UN says it will only do that at the invitation of Jakarta - an invitation, which is unlikely to be made

For its part, the UN is determined it will not abandon the ballot altogether.