Iraqi Prime Minister Nouri al-Maliki hinted at such an operation as recently as June, following successful military operations in Basra in the oil-rich south and in the Baghdad slum of Sadr City. A successful campaign in Diyala will add credence to the growing chorus of voices among Iraqi lawmakers lobbying for a U.S. troop withdrawal.

Diyala is an ethnically diverse region, with Christians, Kurds, Sunnis and Shiites all settling in the region. The terrain makes it advantageous for insurgents, and observers see it as a major throughway for Iranian arms shipments to Iraq, The Daily Telegraph said.

"Diyala remains the most dangerous province in Iraq," said Iraqi military Col. Ali al-Karkhi.

Karkhi said the diversity of the region contributes to the restive nature of the eastern province.

"The other provinces are far less mixed, which is why it is so difficult to restore peace here. It is also the reason why people are so extremist," he said.

The Diyala operation is expected to target Shiite militias operating in the area. Iraqi Interior Ministry spokesman Maj. Gen. Abdul-Karim Khalaf on July 14 said advance military teams had deployed to the region in anticipation of the assault.

"There will be fighting," he said. "It is going to be tough."