Baghdad  - Hundreds of thousands of Shiites from all over Iraq flocked Thursday to Karbala to take part in a religious festival amid tight security.

"Everything is continuing normally and we are working to secure all visitors from Iraq and from other countries and ensure their safe return to their families after performing the visit," First Deputy Governor of Karbala, Abbas al-Moussawi, was quoted by Aswat al-Iraq news agency as saying.

Shiites are heading to Karbala to celebrate the mid-Shaaban visit, which falls on Friday. Shaaban is the month that precedes the holy fasting month of Ramadan.

A man and his wife were killed Thursday by unknown gunmen who opened fire at their vehicle in the northern Iraqi province of Diyala, a local security source said.

The gunmen escaped after opening fire at a civilian vehicle on a main road south of Khanaqin, which lies 155 km northeast of Diyala's capital city, Baaquba.

In Mosul, two gunmen were killed and a third was wounded when the bomb they were carrying to plant on the road exploded in their car. The police carried the two bodies and arrested the wounded for investigation, a security source said.

Earlier on Thursday, Iraqi forces arrested 15 wanted people during search and raid operations, which started Wednesday evening in several areas in eastern Mosul such as al-Karama and al-Intissar. The operation is part of the campaign that started last June to help cleanse the city of gunmen and suspects, the source added.

"The security operations were divided into two phases, during which we were able to arrest dozens of wanted people and redeployed military units to cover all areas under our responsibility," said the source.

Mosul, some 400 kilometres north of Baghdad, is among the most ethnically and religiously diverse areas of Iraq, and remains the site of near-daily fatal attacks.

On Sunday, two policemen were killed when a bomb planted by the side of the road exploded as they patrolled al-Yarmuk, just south of Mosul. A third policeman died and four others were injured when "unknown gunmen" sprayed their patrol with automatic weapon fire in eastern Mosul.