(Updates with fresh monitor, rebel and analyst comments, revises death toll)

By Simon Gardner

COLOMBO, July 10 (Reuters) - A roadside bomb killed five people, including Tamil Tiger insurgents, in an apparent ambush in rebel-held territory in Sri Lanka's far north on Tuesday, Nordic truce monitors said.

The attack in Mankulam, along the island's main north-south artery that spans government and rebel territory, is the latest in a spree of roadside bombs attacks in recent months, the vast majority of them against government troops.

The unarmed Sri Lanka Monitoring Mission initially reported eight Tigers were killed.

"We are hearing contradictory reports, but it seems now that five were killed," a spokesman for the monitors said. "It was a Claymore attack."

The Liberation Tigers of Tamil Eelam said a medical services team comprising rebel fighters and civilian medical personnel was targeted by Sri Lankan troops and that four people were killed and two injured.

Military analysts say deep penetration units of soldiers are operating behind Tiger lines using guerrilla tactics to fight the insurgents on their own terms, and suggest a new chapter in a two-decade civil war will escalate further.

"It was an attack by a deep penetration unit of the Sri Lankan army," rebel military spokesman Rasiah Ilanthiraiyan said by telephone from the northern rebel stronghold of Kilinochchi. "It was a medical unit going to attend to medical services in a village. Four were killed on the spot."

"This is not the first," he added. "There is a cordon and search operation going on throughout the area to stop the perpetrators."

The Sri Lankan military was not immediately available for comment.

Analysts say both the military and Tigers tend to overstate enemy losses and downplay their own, and said the ambush behind Tiger lines was a worrying sign that troops were paving the way for a push north.

"Commando groups are operating in small numbers behind enemy lines because the military has been finding it difficult to break through rebel defences," said Iqbal Athas of Jane's Defence Weekly.

"This is a softening up of targets in the north to facilitate a military advance. It means heavy fighting will shift from the east to the northern theatre."

With land and sea battles, ambushes and killings near daily -- an estimated 4,500 people have been killed since last year alone -- Nordic monitors overseeing the remnants of a 2002 ceasefire which still holds on paper but has collapsed on the ground have given up counting truce violations by both sides.

Analysts say that while the rebels have lost vast swathes of land that they controlled in the east in fierce fighting in recent months, they see no winner on the horizon to a conflict that has killed nearly 70,000 people since 1983.

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