BAGHDAD, Iraq -- About 1,000 American troops, supported by combat helicopters and armored vehicles, stormed into a village in northwest Iraq near the Syrian border on Saturday in an operation aimed at freeing the area from insurgent control and stopping the flow of weapons and fighters from Syria as the constitutional referendum approaches.

The mission focused on Sadah, a village of 2,000 residents that has become an insurgent stronghold in recent months, with militants loyal to Abu Musab al-Zarqawi carrying out killings of local people and government officials, military officials said.

The American military operation in Sadah was the latest of several large-scale sweeps over the past few months in Anbar province, a remote desert region in northwest Iraq that is an important transit point for insurgents smuggling in weapons and fighters across the porous border with Syria. At least eight insurgents were killed in a series of firefights in and around Sadah, military officials said.

Insurgent attacks have left more than 200 people dead across Iraq over the past week, and American and Iraqi military officials have made clear that they expect to see more violence as the militants press efforts to disrupt the Oct. 15 referendum on the new Iraqi constitution.

But no matter how the vote goes, several senior American officials have said in interviews, the violence in Iraq is likely to increase significantly in coming weeks.

That prediction stood in contrast to the upbeat previous assessments from President Bush and others in his administration before other major turning points in Iraq, like the transition to Iraqi sovereignty in 2004 or the national elections early this year.

Insurgent violence continued across Iraq on Saturday. In the northern city of Kirkuk, a roadside bomb struck a police patrol, killing three officers and injuring four, police officials said.

Two American soldiers were killed Saturday, military officials said. One died when his patrol struck a mine north of the capital near Bayji, and another was killed by a roadside bomb in Baghdad.

Also on Saturday, a Danish soldier was killed in Basra when a roadside bomb struck his patrol, military officials said.

The American military also released 500 prisoners from Abu Ghraib prison, part of a planned release of 1,000 detainees in honor of the Muslim holy month of Ramadan, which begins Tuesday.

Four gunmen kidnapped Abdul Jabar Solagh, the brother of the interior minister, in the Sadr City neighborhood of Baghdad on Saturday night, Interior Ministry officials said.