Iraqi police say insurgents have killed at least 40 people and wounded dozens more in attacks just four days before a national referendum on a new constitution.

In the deadliest attack Tuesday, a suicide car bomber struck a crowded market in the town of Tal Afar, near the Syrian border, killing at least 30 people and wounding 45 others. In Kirkuk, at least three people were killed in a drive-by shooting, and in Baghdad a suicide car bomber killed at least seven people at an Iraqi army checkpoint.

U.S. and Iraqi officials have repeatedly warned that insurgents - many of whom are Sunni Arabs - will step up attacks to undermine Saturday's constitutional referendum.

Shi'ite and Kurdish political leaders have been pursuing last-minute negotiations with Sunni political leaders who are concerned their community will be sidelined under the constitution's proposed federal system.

Some information for this report provided by AFP and AP.