Mark Dodd, Dili – Australian and New Zealand peacekeepers based along East Timor's border with Indonesia have come under sustained fire from suspected militia in the most serious test so far of United Nations military readiness after the departure of Interfet.

The military spokesman for the UN Transitional Administration in East Timor (UNTAET), Lieutenant-Colonel Brynjar Nymo, said four incidents involving several hundred rounds being fired, including the targeting of an Australian Army helicopter, had been reported in a 48-hour period to yesterday.

The peacekeeping force believed three of the incidents were "a deliberate and co-ordinated effort", he said. "We are concerned at the fact that apparently groups of militia freely can cross the border without the TNI [Indonesian military] being able to stop this activity. We are concerned that if the TNI cannot take necessary measures the situation may well escalate."

The sudden appearance of modern automatic weapons in the hands of pro-Jakarta militia came only a day after Indonesia's President Abdurrahman Wahid visited Dili, where he gave assurances that militia were being disarmed. Colonel Nymo warned Indonesia that UN peacekeepers operated under a Security Council mandate that allows soldiers to return fire if threatened.

Pro-Jakarta militia are thought to be responsible for the four incidents which occurred on Wednesday and Thursday. "We have had an increased number of reports over the past three or four weeks indicating a larger number of militia being active," Colonel Nymo said. Asked if TNI could have been involved, he said: "I would not go into speculation, but we have no indications that they are."

The first of the incidents occurred at Motamoruk, near the north-coast border town of Batugade, on Wednesday. An Australian observation post near the village of Motamoruk had reported hearing 30 to 40 rounds being fired nearby, Colonel Nymo said.

In the next incident, New Zealand troops at Belulik Leten, near south-western Suai, had received "harassing fire" from a border crossing point.

An hour later an Australian Army border observation post came under fire at Memo, near western Maliana, and on Thursday night an Australian Army Kiowa light observation helicopter was fired at from Indonesia while on a reconnaissance mission near the border. Colonel Nymo said the helicopter was not hit, and there was no return fire in any of the incidents.

Before leaving East Timor last week, the Interfet commander, Major-General Peter Cosgrove, had expressed concern that pro-Jakarta militia would test the capacity and resolve of the UN force that took over from Interfet.