Military chiefs in Afghanistan say that they are expecting a spike in violence as international forces, encouraged by a US boost in troop numbers, move into the spring fighting season.

US President Barack Obama has announced an extra 21,000 US troops together with a new strategy to tackle Islamic extremism in this region, for which he will seek support from his European allies at this week's NATO summit.

The reinforcements will be a "game changer," the Dutch commander of 23,000 NATO troops in the restive south, Major General Mart de Kruif, told reporters here.

It would enable the NATO-led International Security Assistance Force (ISAF) to expand its operations and increase the pressure on insurgent leaders, the drugs trade, and bomb-making cells, he said.

That will likely lead to a "significant spike in incidents" as the rebels react, he cautioned, but would sow the seeds for a significant improvement -- next year.

The problem up to now, ISAF spokesman Brigadier General Richard Blanchette told AFP, was that there were simply not enough soldiers to capture and hold insurgent areas in the south.

"You just run out of troops," the Canadian said, adding that reinforcements due to start deploying from late spring had created a "positive mood."

"We will see more incidents occur because we will go into areas where until now they had safe-havens and they did their smuggling and violent acts."

Last year's violence was the worst since the US-led invasion of Afghanistan in late 2001 following the September 11 attacks in the United States by Osama bin Laden's Al-Qaeda group.

The invasion drove the Taliban from power in Kabul, but they and Al-Qaeda, whom they were sheltering, have since been able to regroup, locking down and harassing tens of thousands of international troops here.

Around 4,000 of the extra US troops will work on training Afghan security forces so they can take on a larger role in the bloody fight against Taliban and other insurgents.

The other 17,000 will boost NATO-led forces in the south, where insurgents are tied into a lucrative narcotics trade.

Those are on top of nearly 70,000 foreign troops from 42 countries serving under NATO or US command.

So far this year, 77 foreign soldiers have lost their lives in Afghanistan, most of them in attacks, according to the icasualties.org website that tracks the conflicts here and in Iraq.

Its figure for the whole of last year was 294.

Improvised explosive devices account for 65-70 percent of casualties to the force but claim four to five times as many Afghan civilians, Blanchette said.

Security forces are targeting the networks that make the devices. "In that we are making progress."

IEDs are "absolutely now the weapon of choice by the insurgency," De Kruif agreed, and a highlight of planning for the new US troops is that there would be enough armoured vehicles to better protect them.

Blanchette said the situation was complicated by a difficulty in precisely defining the Taliban, who include a hardcore, religious segment as well guns for hire.

"We know that a lot of them are farmer by day, fighter by night," he said, "and this is making it very difficult for any specific operations to clear an area completely."

International forces also have to overcome Taliban propaganda and an often sceptical public alienated by civilian casualties caused by the military.

Winning the participation of the "huge mass" of Afghans who do not support the insurgents is vital to the "three-pillar" counter-insurgency strategy of building security, governance and development, Blanchette added.

This week's summit is expected to renew international commitment to Kabul, despite concern in some nations over the mounting financial and human cost.

Obama's pledge to ramp up the civilian effort here has been praised. While Washington has dropped appeals for more soldiers from Europe, he is likely to call at the summit for civilian expertise.

The political commitment outweighed the troops crunch facing the military, Blanchette said.

"The challenges are huge but at the same time the will of the international community is quite huge," he said.