KABUL — Canada's chief of defence staff is in Afghanistan to spend Christmas with Canadian soldiers serving with the NATO force providing security to the Kabul government.

Gen. Ray Henault arrived under heavy security Wednesday at Camp Julien, the main Canadian base in the Afghan capital, for the surprise visit.

Henault said he came to Kabul to say "thank you" from Canada to the troops serving as part of NATO's International Security Assistance Force, or ISAF. Canada has nearly 2,000 soldiers working with ISAF in and around Kabul.

Many of those soldiers were jolted out of bed by a blast early Christmas morning, which could be heard across much of the Afghan capital.

The explosion occurred at 5 a.m. local time in a district of Kabul that's patrolled by Italian troops.

The Associated Press reported a bomb had exploded outside a house used by United Nations staff in the capital, demolishing part of a wall surrounding the building site and shattering several nearby windows but causing no injuries.

The blast occurred at least eight kilometres from the Kabul university where an historic constitutional council is taking place.

Gen. Baba Jan, the Kabul police chief, said the explosion was caused by a bomb.

"It was a bomb," Jan said from the site of the explosion.

"This is the work of the enemies of Afghanistan."

Four men emerged from the two-storey building in a residential area in the south of Kabul after the blast.

"We were all sleeping. Then we heard this explosion," said Ahmed Wurie, a UN staffer from Sierra Leone working to prepare for next year's elections in Afghanistan.

Wurie said he and his three colleagues were shaken but unhurt.

Two other UN staffers who normally stay in the house were out of the country at the time of the blast.

The explosion highlights the dangers faced by Canadian and other international soldiers in Afghanistan -- even on Christmas Day.

Afghanistan is a Muslim country, and does not celebrate the holiday.

Roughly one-half of the Canadian soldiers in the war-torn country will celebrate Christmas with a turkey dinner.

The rest had their dinner Wednesday and will be out on patrols, treating Christmas as just another regular work day.

Henault's itinerary includes a tour of a refugee camp and a visit to soldiers at Camp Warehouse, the smaller of the two Canadian bases, in another district of Kabul.

The general arrived shortly after troops began celebrating Christmas.

A simple blessing marked the beginning of Christmas celebrations Wednesday. Hundreds of soldiers were treated to turkey and all the trimmings at the first dinner at Camp Julien.

As part of military tradition, the oldest and youngest soldiers at the dinner got to sit at the head table and temporarily exchange ranks with their commanding officers.

Later, soldiers sang carols during an early evening Presbyterian mass.

Padre Barb Putnam said going to the chapel at Camp Julien wasn't quite the same as sitting beside loved ones in a church at home on Christmas Eve.

But she said the soldiers are a family too and they gather around one another more than usual at this time of year.