BAGHDAD, Iraq -- Bomb blasts killed six Marines as the U.S. military announced Friday it had completed a major sweep in western Iraq aimed at suppressing al-Qaida militants before next week's vote on Iraq's constitution.

The military said 50 insurgents were killed in the six-day offensive, launched Oct. 1 in towns near the Syrian border. The operation was the first in a series of major offensives in the past week in the heartland of the Sunni-led insurgency.

U.S. forces have swept through the area before -- most recently in May -- but militants have always returned, bringing in foreigners from Syria and planning attacks to be carried out elsewhere. The military said they will now leave a long-term presence there.

The military has said it will wrap up the operations in time for Sunni Arabs in the region to vote in the Oct. 15 referendum, a key concern of Sunni leaders who have threatened a boycott.

Eight days before Iraqis were to approve or reject the draft constitution, most were still waiting for copies of it to read. Distribution began in a few Baghdad neighborhoods, but did not appear to have begun elsewhere.

Campaigning has begun in earnest, however, with radio and television running call-in and talk shows on the document, which has strongly divided Iraq's Shiite majority and Sunni minority. Shiite and Kurdish leaders support the constitution -- whose passage the United States is eager to see -- while Sunnis denounce it, saying it will split the country.

Walls around Baghdad were plastered with posters advertising the referendum. One depicts a white-veiled woman over the slogan ''Our dreams are greater than their threats,'' seeking to overcome fears of insurgent retaliation attacks against those who vote.

In a boost to the pro-constitution side, radical cleric Muqtada al-Sadr -- one of the few influential Shiite leaders to speak out against the document -- gave his followers approval to vote ''yes.''