The US military said earlier this month that a force of 30,000 Iraqi soldiers and police would take part in the offensive, although of local official has said nearly 50,000 personnel are involved in the US-backed operation.
'Law will be imposed'
Ibrahim Bajilan, the head of the regional council, told The Associated Press news agency that the crackdown would last for about two weeks and then "law will be imposed in all Diyala".
The province, which has a long border with Iran, has been a centre of weapon smuggling, has suffered repeated suicide bombings and controls key supply routes to Baghdad and the northern cities.
Brigadier General David Perkins, US military spokesman, said the main challenges in Diyala were the hot summer temperatures and the abundance of hiding places and enclaves in palm groves and other agricultural areas.
"It just takes a lot of physical effort to go through this difficult terrain in very hot and demanding conditions," he told a joint news conference with al-Askari.
Iraqi forces moved into the border area late last week and have unearthed weapons caches of mortar rounds, a rocket propelled grenade launchers, and AK-47 rifles.
"The operation started south of Balad Ruz to deny the area as a safe haven to criminals in the Diyala and Baghdad provinces and to pursue them where ever they may go," Lieutenant Colonel Paul Calvert of the US army said on Wednesday.
"Operations will continue to press forward until the objective has been accomplished."
The offensive follows similar Iraqi military operations in the southern provinces of Basra and Maysan, and the northern province of Nineveh, as Iraq forces try to shore-up recent security gains.
On Wednesday, an independent website which keeps records of US military casualties in Iraq said that the number of American soldiers killed in combat had dropped dramatically.
Five US soldiers have been killed in combat in Iraq so far in July compared to 66 in the same month last year, according to icasualties.org.
The US combat death toll in July is also down from 23 soldiers in June and 15 in May, according to the website.