NATO forces in southern Afghanistan, boosted by a big influx of U.S. troops, will step up operations in Helmand province and the city of Kandahar soon, the top regional commander said on Thursday, Reuters reported.

"We are entering a new stage in the operation," Dutch Army Major General Mart de Kruif told reporters at the Pentagon in a conference call from NATO's base at Kandahar Air Field.

The United States has been pouring thousands of troops this year into southern Afghanistan, the heartland of the Taliban movement and scene of the heaviest fighting, to prepare for an effort to regain the initiative from the insurgents.

De Kruif said security incidents had decreased in two provinces in his region -- Uruzgan and Zabul.

"The insurgency is more or less is forced to concentrate its efforts in Helmand and in Kandahar," he said.

"In a very short time ... we will start significant operations ... to secure central and southern Helmand and stabilize Kandahar," de Kruif said.

"We will have the operational initiative on our side and maintain it," he said.

De Kruif said the new offensive by NATO forces would lead to a spike in violence but the prime objective would be to protect the population from insurgents rather than hunt down and kill Taliban fighters.

The Obama administration has declared Afghanistan its top military priority and the United States is more than doubling its military presence in the country to around 68,000 troops, from 32,000 at the end of last year.

The United States now has 57,000 troops in Afghanistan and other nations, mainly NATO allies, have some 32,000 there.

De Kruif estimated there were between 10,000 and 18,000 Taliban fighters in southern Afghanistan.

They could be broadly divided into three groups, he said -- religious hardliners linked to the Taliban leadership in the Pakistani city of Quetta, fighters linked to the narcotics trade and so-called "$10-a-day Taliban" motivated by money.

De Kruif said NATO and Afghan forces could succeed in establishing security in southern Afghanistan "in a couple of years" and also significantly improve conditions before the country's presidential election on August 20 this year.

"At the end of the day, it's good governance and reconstruction and development that will bring lasting success," he added.

He said the best indicators of success he had found were the number of shops open in the bazaars and the number of schools open in the villages -- signs that commerce could flourish and people could live freely.