Kandahar, Afghanistan ? Islamic militants, some armed with machine guns, battled Afghan, U.S. and Canadian forces and exploded two suicide car bombs Thursday, some of the deadliest violence in Afghanistan since the fall of the Taliban.

More than 100 people were killed in the string of attacks that started Wednesday: dozens of insurgents, at least 15 Afghan police, an American civilian training Afghan forces, and the first female Canadian soldier to die in combat.

The fighting concentrated in the southern provinces of Helmand and Kandahar raised new concerns for the future of Afghanistan’s fragile democracy. The Taliban have stepped up attacks in recent months, with roadside bombs and suicide assaults, but this week’s fighting marked an escalation in a region where the U.S.-led coalition is to cede control of security operations to NATO by July.

President Hamid Karzai said the violence emanated from the mountainous border trail regions of neighboring Pakistan, populated by the ethnic Pashtuns who make up the majority of the Taliban militants and are believed to be hiding Osama bin Laden.

“We have credible reports that inside Pakistan, in the madrassas, the mullahs and teachers are saying to their students: ‘Go to Afghanistan for jihad. Burn the schools and clinics,”‘ Karzai said.

Foreign security guards stand near the scene where a car bomb exploded in Herat, in western Afghanistan. Suicide attackers detonated car bombs Thursday in two Afghan towns, Herat and Kabul, killing an American working on a counternarcotics project and an Afghan civilian, officials said.

Pakistan’s Foreign Ministry spokeswoman, Tasnim Aslam, called the allegations “baseless.”

The violence started Wednesday in the small remote town of Musa Qala in Helmand, when an estimated 300 to 400 militants with assault rifles and machine guns attacked a police and government headquarters.

The attack sparked eight hours of clashes with Afghan security forces, the fiercest in Helmand since U.S.-led forces ousted the Taliban in 2001 for hosting al-Qaida, said Deputy Gov. Amir Mohammed Akhunzaba. He said the fighting started at 10 p.m. Wednesday, though the Interior Ministry said it was earlier.

He said the bodies of about 40 Taliban militants were recovered and that 13 police were killed and six wounded in the fight, some 280 miles southwest of Kabul.

Afghan police reinforcements forced the militants to flee. British soldiers helped evacuate casualties but did not provide military backup, in part so Afghan forces could prove themselves, said British military spokesman Capt. Drew Gibson.

“If they’re the ones who are seen beating off the Taliban, there’s a lot of credibility for them,” Gibson said. “The ANP (Afghan National Police) did admirably in the circumstances, proven by the fact that Musa Qala is now back under ANP security.”

In Afghanistan’s wars, violence typically increases in the spring, after the melting of winter snows that hamper guerrillas’ movements. But to judge by propaganda distributed in Afghanistan and Pakistan, Taliban militants seem to feel they have a special opportunity to win ground this year and even perhaps to push Western troops out of parts of the country.

The Taliban’s optimism may be linked to changes in the forces opposing them. U.S. forces in the south are handing over combat roles to a mix of British, Canadian and Dutch soldiers who are now arriving. By June 30, the U.S. Central Command, which currently oversees all combat activities by coalition forces, is to hand that authority in the south of Afghanistan to NATO.