Afghanistan's surging violence left three Canadian soldiers and 21 suspected Taliban militants dead Wednesday, as insurgents grabbed control of another district headquarters in the south.

NATO said it faced a seasonal upsurge in militant operations, but dismissed recent suicide and bomb attacks as "militarily insignificant."

The Canadian troops died when their vehicle was struck by an improvised explosive device as they were on a supply mission between two checkpoints in Kandahar's Panjwayi district, said Brig. Gen. Tim Grant, the head of Canadian forces in the country.

The latest deaths brought to 60 the number of Canadian troops killed in Afghanistan, since deploying to the country as part of the NATO-led force in 2002.

NATO and Afghan troops, meanwhile, clashed with militants in the same province and called in airstrikes, killing 21 suspected fighters, local mayor Khairudin Achakzai said. The militants bodies were strewn in the battlefield along with their weapons and ammunition, he said.

Violence this year has left over 2,400 people — mostly militants — dead, according to an Associated Press tally of figures from Western military and Afghan officials.

The recent spike in insurgent attacks and bombings shows the Taliban remains unbowed more than five years after its ouster, despite the deaths of thousands of militants.

"We find ourselves in the midst of the so-called fighting season, when what we had predicted is taking place: an increase in suicide bombings and more desperate attempts by the enemies of peace and stability to present the illusion that they are stronger than they are," said Lt. Col. Maria Carl, spokeswoman for NATO's International Security Assistance Force.

She described the attacks as tragic but "militarily insignificant."

Militants, meanwhile, pressed ahead with their tactic of taking control of remote districts, an apparent attempt to stretch thin the Afghan security forces.

In the northern tip of Kandahar province, police lost control of the remote Ghorak district to the militants, hours after retaking a neighboring district of Miya Mishin that had recently fallen to the militants, said Esmatullah Alizai, the provincial police chief.

Those insurgent actions come against the backdrop of bitter fighting in neighboring Uruzgan province, where fierce clashes between Taliban militants and Afghan and NATO forces have killed more than 100 people since Saturday, including dozens of civilians, according to at least two Afghan officials.

In Kabul, dozens of mourners filed into the mosque in central Kabul to pay respects to the families of police killed in a Kabul bus attack on Sunday — which left 21 policemen and a number of civilians killed — the deadliest insurgent attack since the fall of the Taliban in late 2001.

"Twenty-one of our teachers were killed in the blast that day. It was a black day for police. I pray that God punish these terrorists, and that terrorism will fail," said Abdul Aziz, one of the dozens of police officers in navy blue uniforms at the wake.

Mourners walked past portraits of the dead policemen hanging in front of the walls and windows of the mosque. They walked up to family members, placed their hands on their hearts and bowed solemnly in a quiet procession. "God bless them," they said, one by one.

In other violence:

— Assailants ambushed a convoy belonging to U.N.'s Office for Project Services on the main Kabul-Kandahar highway, killing two Afghan guards and wounding another, said Jailani Khan, highway police chief for Zabul province.

— Two gunmen opened fire on people praying in a mosque Tuesday evening, killing three and wounding four others, in Ismail Kheil, a village in Khost province, said Wazir Pacha, a provincial police spokesman.

— Also in Khost, a roadside bomb hit the vehicle of the Qalandar Khel district chief, killing him and his driver Wednesday afternoon, said provincial police chief Gen. Mohammad Ayub.