The United Nations has criticized a move by Iraq's Shi'ite and Kurdish legislators to change the rules for this month's constitutional referendum, making it harder for the document to be defeated.

U.N. officials in Baghdad said Tuesday they have expressed their concern to the national assembly and the government.

The rule changes, passed quietly on Sunday, virtually ensure the success of the constitution, but U.N. officials contend they call into question the fairness of the referendum.  Under the new rules, the constitution can only fail if two-thirds of registered voters, rather than actual voters, vote "no" in at least three of Iraq's 18 provinces.

That move also has angered the minority Sunni Arab community, which is threatening to boycott the October 15 vote.

The political haggling comes as 2,500 U.S. forces launched a new offensive against insurgents in three major towns along the Euphrates River.  The operation, called River Gate, coincides with a similar one continuing near the Syrian border.

Some information for this story provided by Reuters.