Vote counting in Iraq's constitutional referendum continued Sunday, with spot checks of preliminary results suggesting the U.S.-backed charter may have passed.

The Associated Press says an early vote count in crucial Diyala province showed 70 percent of voters saying "yes" to the constitution. And with most ballots counted in Ninevah province, AP reports "yes" votes were outnumbering "no" nearly four to one.

Diyala and Ninevah are two of four key provinces where Sunni Arab opponents were relying on residents to defeat the constitution. The charter will pass with a simple majority, but will fail if two-thirds of voters in any three provinces reject it. More results are expected later today and Monday.

The U.S. military says that five American soldiers were killed by a roadside bomb in the mostly Sunni city of Ramadi during voting on Saturday, but otherwise said the day was relatively peaceful.

Some information for this report provided by AFP and Reuters.