Iraq's Christian minority said they were too afraid to go out for the traditional Chistmas eve mass Wednesday as they worried about violent crime and the deadly war between US forces and insurgents in the capital.

Explosions and loud automatic fire jolted southern Baghdad early Wednesday, at least some of which the military said was part of an ongoing operation called Iron Justice.

The fire marked the start of "phase two" of Iron Justice, which a military source described as the operation's "kinetic phase" after soldiers had gathered intelligence on their enemy.

The explosions, which sounded like artillery fire, began about 00:30 am (2130 GMT) and carried on for about 20 minutes, laced with bursts of automatic weapon fire near the southern suburb of al-Dura. Aircraft buzzed overhead.

The US military had just cancelled an invitation for media to observe artillery fire Wednesday on positions from where they believed insurgents were shelling coalition forces in Baghdad.

Even before the early morning blitz, Baghdad's Christians expressed fear about going to midnight mass or the usual parties to celebrate their first Christmas without Saddam Hussein.

"We are afraid of explosions," said Nasreen Thomas, 30, a dentist. "Under the old regime we celebrated until the wee hours but this year we can't ... Maybe I won't go out of my house."

North of Baghdad, US troops searched Mosul, still home to many Baath supporters, for clues to the whereabouts of Saddam's top henchman Izzat Ibrahim.

Soldiers arrested a childhood friend of Ibrahim, considered one of the masterminds of the deadly anti-US insurgency, relatives and an Iraqi security force member said.

The aide of Ibrahim, identified as Sheikh Ghazi Hanash, head of the influential Tayy tribe, was detained at his Mosul home along with three of his sons, said Hanash's relatives and Waadallah Tewfik Hassan, an Iraqi Civil Defence Corps member who participated in the raid.

A firefight ensued that left one of the sheikh's bodyguards dead and his daughter wounded.

Hanash's home was marked by bullet holes and broken glass.

Ibrahim, Saddam Hussein's number two in the Baath party, is suspected of forging an alliance with Islamic extremists in battling the eight-month-old US-led occupation.

Following Saddam's arrest on December 13, Ibrahim, who reportedly suffers from leukemia, is the most senior former Iraqi official still at large.

The US military has put a 10-million-dollar price on his head and is eager to catch the last of Saddam's cronies and silence the resistance once and for all.