When international peacekeepers landed here last month, the militiamen who opposed independence for East Timor mounted little resistance. Many retreated to the hills or over the border into Indonesian-controlled western Timor, allowing the peacekeepers to move quickly through the territory.

Now, weeks after regrouping and reassessing the situation, militia members are becoming more confrontational, engaging in gun battles with patrolling soldiers and raising the prospect that the mission of the Australian-led force will be tougher and riskier than expected.

In the bloodiest confrontation to date, a band of about 20 militiamen fired on peacekeepers setting up an observation post today near the village of Marko, about 10 miles from western Timor. The peacekeepers fought back for more than an hour--killing three militiamen and wounding three others--before being evacuated by helicopters, officials said. None of the peacekeepers was injured.

Today's clash was the fourth in the past 10 days. Last week, a small patrol of peacekeepers was drawn into a firefight near the western Timor border that left one militiaman dead. The next day, Australian troops were involved in a border skirmish with militias in which an Indonesian policeman reportedly was killed. And on Oct. 4, two Australian soldiers were wounded and two militiamen were killed near the town of Suai.

"We could have these episodes for quite some time," Australian Maj. Gen. Peter Cosgrove, the commander of the 7,000-member multinational force, said in an interview today. Such confrontations could become more likely, he said, as peacekeepers increase their presence in the western parts of East Timor, where some militiamen have been holed up since the foreign troops began arriving on Sept. 20.

"We're seeing a new degree of aggression" by the militias, said Col. Mark Kelly, Cosgrove's chief of staff.

Indonesia permitted the peacekeepers to enter East Timor last month to stop a rampage by the Indonesian army and the militias after residents voted overwhelming to split from Indonesia, which invaded the former Portuguese colony in 1975 and annexed it the next year.

Although many militia members fled to western Timor, where they reportedly have set up training camps, peacekeeping officials play down the prospect of a large-scale militia invasion into East Timor. Instead, they say the militias will continue to engage in small, targeted confrontations and border incursions.

Cosgrove disputed an Associated Press report that said about 150 militiamen, including leader Eurico Guterres, held a rally last week in Liquica, a village about 30 miles west of the East Timor capital of Dili, where the peacekeeping force is based. Cosgrove, citing his own intelligence reports, said such a gathering never occurred.

At the same time, he said, the peacekeepers have not been able to prevent militia members, who don't wear uniforms, from crossing the border. "It's a giant border," he said. "I can't say it's impermeable."

Cosgrove said he eventually would like to "reduce the chances for confrontation" by setting up a demilitarized zone along the border, which would be monitored by unarmed U.N. observers.

Indonesian officials have strongly condemned last week's border skirmish that killed a policeman, contending that the Australian soldiers had crossed into western Timor when the shooting took place.

But an Australian military spokesman said today that an investigation by the peacekeepers and the Indonesian military had established that the Australian soldiers were inside East Timor during the incident.