US soldiers killed at least four Iraqi civilians yesterday in a town at the heart of guerilla attacks, witnesses said, while a US helicopter crashed nearby after probably being hit by ground fire.

Iraq's US Governor Paul Bremer said guerilla attacks in the country had fallen dramatically and plans to hand over power to Iraqis by the end of June were well under way.

Diplomats in New York said the United Nations planned to send a small team of security advisers to Baghdad to assess safety conditions in preparation for a possible return of international UN staff to Iraq.

A US Army foot patrol came under rocket attack in the town of Falluja, west of Baghdad, after a noisy anti-American protest and the soldiers killed at least four civilians when they returned fire, including an elderly woman, witnesses said.

Hundreds of people took part in the demonstration, called to protest against the arrest of a young woman by US troops in Falluja on Monday. Speakers said the woman was alone at home at the time and that her arrest was an insult.

"The young mujahideen (holy warriors) promised to take their revenge (over the arrest)," said Omar Sabah, a worshipper at a mosque, referring to masked armed youths who roamed the streets on Monday after word spread about the woman's detention.

Khalas Ahmed, a 15-year-old boy selling cigarettes from a kiosk, said he witnessed the attack on the US soldiers and saw two rockets fired at them.

"The Americans started firing back. The bullets hit my kiosk and I dived to the ground," he said.

The woman who was killed was on a balcony of a house and a passing car was caught in a hail of bullets which killed all three men inside, other witnesses said.

The US military had no immediate information on the incident. Falluja, some 50 kilometres from Baghdad, is in the so-called Sunni Muslim "triangle" where most guerilla attacks have occurred since US-led forces toppled Saddam Hussein in April. Saddam, a Sunni, had strongholds in the triangle.

The US military said both crew members survived the crash of the Apache helicopter near Thirthar lake, north of Falluja.

"Our initial information tells us that it's possible that the (Apache) helicopter was downed by, or at least was struck by, enemy fire," said a spokesman.

At least three US helicopters have crashed around Falluja in less than two weeks, killing 10 military personnel.