Iraqi police say bomb blasts in Baghdad have killed at least 70 people and wounded dozens more.

In the deadliest attack, at least 50 people were killed and another 60 wounded when a suicide bomber detonated a fuel tanker near a gas station in the capital's western Mansour neighborhood.

Also Wednesday, Iraq's largest Sunni Arab political bloc announced it is withdrawing from Prime Minister Nouri al-Maliki's coalition government, further undermining attempts at national reconciliation.

The Sunni Arab Iraqi Accordance Front, Rafaa al-Issawi, said Mr. Maliki's Shi'ite-majority government had failed to meet its demands and the bloc's six Cabinet ministers would submit their resignations Wednesday. The demands included dealing with Shi'ite militias and reforming the Iraq security forces and detention procedures.

The Accordance Front said its 44 members of parliament would remain in the 275-seat legislature.

In Washington, the White House downplayed the significance of the withdrawal. A spokesman said the bloc is not withdrawing completely from the political process, and that some Sunni Arabs remain in top positions.

The U.S. military said Iraqi and coalition forces discovered a mass grave in Diyala province, east of Baghdad. Residents said al-Qaida in Iraq killed about 17 villagers, including the elderly and children, last month.

The U.S. military also announced the combat deaths Tuesday of four American soldiers, including three killed by a sophisticated armor-piercing bomb.

U.S. officials have accused Iran of supplying such weapons to insurgents in Iraq, but Iran denies the charge.

Some information for this report was provided by AP and Reuters.