(Recasts, adds comments from military commanders)

By Y.P. Rajesh

KABUL, March 25 (Reuters) - NATO commanders said on Sunday they were pleased with the initial results of a spring offensive launched this month to pre-empt the Taliban in their southern strongholds.

The deployment of several thousand NATO and Afghan troops as part of Operation Achilles in the southern province of Helmand -- Afghanistan's poppy-growing heartland -- has put the guerillas on the back-foot, senior officers said.

The comments came hours after Afghan and NATO troops killed 12 suspected Islamist insurgents overnight when they tried to attack a military base in the southeastern province of Paktika, on the border with Pakistan.

The attackers were repulsed with small arms fire, backed by air support and artillery, a statement from the coalition said. Two coalition and two Afghan soldiers received minor wounds.

"Achilles is delivering positive results and our efforts have resulted in eroding the enemy's capacity to fight," said Major-General Ton van Loon, the southern command chief of NATO's International Security Assistance Force.

Troops had encircled the fighters and prevented them from getting reinforcements, including foreign fighters from across the border in Pakistan, he told a news conference.

Clashes between the Taliban rebels and coalition troops has escalated with the end of winter in Afghanistan in what is expected to be a crunch year for both sides.

Last year saw the worst violence in Afghanistan since U.S.-led forces ousted the Taliban from power in late 2001. More than 4,000 people died in fighting in 2006, including about 1,000 civilians.

BIGGEST OFFENSIVE

Fighting is expected to be heavy in 2007 as the Taliban have announced they have prepared thousands of suicide bombers.

Achilles, which involves 4,500 NATO troops and 1,000 Afghans, is the biggest operation so far by coalition forces.

It targets the Taliban and drug lords who are reaping record crops for the second year running in Afghanistan, the world's leading producer of heroin.

Last week, 69 Taliban guerrillas were killed in Helmand.

The three-day operation which began on Thursday was the biggest ever by Afghan forces and had cleared a large district to allow for reconstruction and development, the British military command in Helmand said in a separate statement.

"We encountered little insurgent resistance and have established a strong foothold in the areas, which until now contained a high concentration of enemy forces," Lieutenant-Colonel Rob Magowan, said in the statement.

Major-General van Loon said the pressure on the Taliban had resulted in them operating in small groups and being "forced to only do small-scale terrorist attacks".

Afghan forces had arrested a senior Taliban leader this month trying to flee a security operation in the southern province of Kandahar dressed in a burqa, the Dutch commander said.

"That indicates how difficult we have made it for them to operate."

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