Taliban militants and their allies have launched an intensified campaign against thousands of NATO troops deploying to southern Afghanistan, where the multinational force is taking over from U.S. soldiers.

Whether ambushing Afghan police from mountain passes or detonating bombs on lonely highways, remnants of the ousted Islamic regime have stepped up attacks, causing havoc and insecurity across a cluster of provinces.

Military officials and analysts said Wednesday the Taliban threat is the No. 1 challenge facing more than 7,000 U.S., Canadian, British and Dutch troops that by September will be fighting under the NATO flag in four southern provinces.

"This is counterinsurgency warfare (and) there will be casualties on both sides," said British Col. Chris Vernon, chief of staff for NATO forces operating in southern Afghanistan. "This is not the north or west of the country. This is a counterinsurgency war zone."

Taliban chiefs like Mullah Omar hail from southern Afghanistan. Its deserts and mountain ranges provide good cover for militants hiding or planning for attacks. Protecting opium poppy fields — and the illicit funds they earn — is another reason to fight.

Mountains running through the northern districts of the neighboring Helmand and Kandahar provinces, and Zabul and Uruzgan to the north offer sanctuaries for militants, Vernon said. The porous Pakistani border runs along the southern and eastern edge of the provinces, providing another base where militants replenish funds and weapons before sneaking back in to launch attacks.

Ordinary Afghans and foreign analysts are critical that militants can still pose such a threat, more than four years after the late 2001 U.S.-led invasion that ousted the Taliban government for harboring Usama bin Laden and his Al Qaeda terrorists, blamed for the Sept. 11 attacks.

"The situation would have been a lot easier if we got troops down there four years ago," said Joanna Nathan, the Kabul-based senior analyst for the International Crisis Group. "Security has gotten much worse. Four years ago they would have been welcomed, but things have been allowed to fester."

NATO officials believe the militancy will subside in regions such as Helmand, Kandahar and Uruzgan once foreign forces consolidate their presence. Only 1,000 British troops are in Helmand, where no coalition forces have been before, but that number will rise to 2,500 by July. Another 1,500 Dutch troops are due to boost security in Uruzgan by August or September. They are part of an expansion of a NATO-led security force, which is gradually assuming command of all foreign troops in Afghanistan.

"Helmand has been a free zone for the Taliban and the narco-traffickers, but now as coalition-UK capability moves in there, things will improve," Vernon said. "But it is going to take a good year or so to get that sorted."

Extremists are also launching attacks to protect their massive opium poppy plantations from coalition and government efforts to eradicate the crops, which produce 90 percent of the world's opium and heroin.

Tribal disputes, criminal rivalries and anti-Western militants crossing from sanctuaries in Pakistan are also fanning the violence.

Afghanistan has 27,000 new soldiers and another 60,000 lesser-equipped police, many of whom are based in the south. But they still aren't enough to counter strengthening Taliban forces and the more violent tactics, such as suicide attackers and roadside bombs.

"We are afraid when we increase our security presence in the community, we become targets for these terrorists," a Kandahar-based Afghan army commander said on condition of anonymity because of the sensitivity of the issue.

Despite the brazen nature of the Taliban attacks, many of the reported casualties have been on the militant side.

At least 30 militants died in a bold attack in late March on a remote coalition forward operating base in Kandahar. One Canadian soldier was also killed.

Canadian forces killed 20 militants planning an ambush in Helmand province's Sangin district the past weekend. Four Taliban militants were killed Wednesday by police in another foiled Helmand ambush.

But Taliban militants have still been able to inflict casualties, killing four Canadian soldiers April 22 in a village north of Kandahar city. Militants also kidnapped and beheaded an Indian engineer this week in Zabul province, where some 1,500 U.S. soldiers are based.

"We are seeing small cells of never more than 15 to 20 fighting men, occasionally up to 30, operating with local leaders dispersed across the south without great coherence," said Vernon. "This makes them difficult to track."

Key to NATO efforts is its three-pronged approach to supporting security, reconstruction and improved governance.

"If we can get these three lines together, eventually the people will say that they would rather have us than the Taliban," he said.

Kandahar clothing shopkeeper Haji Din Mohammed, 45, said Afghans are desperate for increased coalition support to confront the growing Taliban influence in southern villages and towns. Militants demand housekeepers give them food and shelter at night.

"Outside of the city, everywhere you can easily find the Taliban," Mohammed said. "The government and coalition forces promised us security and an improved economy, but instead the security is bad. I can't go to my nearby village after 5 p.m."