AFP - US Defense Secretary Robert Gates told troops in southern Afghanistan on Tuesday to brace for a tough fight as generals laid plans to battle the Taliban in Kandahar.

Gates is on the second day of a visit to review a surge of US and NATO troops set to bring the number of foreign forces in Afghanistan to 150,000 by the summer in a last-ditch effort to end a war now in its ninth year.

He visited an army battalion that suffered heavy losses last year in the fight against entrenched militants and is based 48 kilometres (30 miles) north of Kandahar, a bastion of the Taliban insurgency.

"Here in the environs of Kandahar, you're in an area that once again is going to be important," Gates told troops from the Fifth Stryker Brigade Combat Team, who have succeeded in pushing back the Taliban in one area.

"Once again you will be the tip of the spear," he said.

His comments were the latest indication of US-led preparations to take on the Taliban in their spiritual heartland.

On Monday, Gates called a major operation under way in the Marjah area of Helmand province "only one of many battles still to come in a much longer campaign focused on protecting the people of Afghanistan".

The overall ground commander, US General Stanley McChrystal, said US and NATO troops could take on the Taliban in Kandahar as early as this summer when enough reinforcements are on the ground. Related article: Kandahar operation to happen this summer

Gates alluded to the heavy losses suffered by the battalion at the outpost last year, with at least 21 killed and 62 wounded in action.

"You've had a very tough tour here," Gates told the troops.

"You came into an area that was totally controlled by the Taliban, you fought for a critical battle space and bled for it."

General David Rodriguez, deputy commander of the NATO-led force in Afghanistan, told reporters travelling with Gates that the battalion would now focus on securing roads around Kandahar.

On Tuesday a roadside bomb ripped into an Afghan police vehicle in the province's Spin Boldak district on the Pakistani border, killing three police officers and a civilian, said Abdul Raqiz, Kandahar's commander of border police.

About 6,000 of the 30,000 additional troops pledged by President Barack Obama in December have arrived in Afghanistan, Gates said, with the rest due to deploy by the end of August. The south is the main focus of the surge.

It is Gates' first visit to Afghanistan since NATO and Afghan troops swept into Marjah, a former Taliban stronghold in the southern province of Helmand, in an assault seen as a pivotal test of Obama's bid to turn around the war.

At Kandahar Air Base, Gates met British Major General Nick Carter, the NATO commander in southern Afghanistan, but as he conferred with officers an alarm sounded for a rocket attack -- a common occurrence on the base.

A few minutes later the all-clear was given.

Gates awarded the prestigious silver star to two army helicopter pilots, including one whose Chinook survived a hit by a rocket-propelled grenade as it was evacuating five wounded soldiers.

The number of coalition forces has been growing around Kandahar, from where the Taliban ruled Afghanistan from 1996 until the 2001 US-led invasion, with about 30,000 troops now deployed in the area.

Arch US foe Iran has confirmed President Mahmoud Ahmadinejad will head to Afghanistan on Wednesday, potentially overlapping with Gates.

Ahmadinejad has repeatedly called for the withdrawal of US-led troops from Afghanistan, while US officials have long accused Iran of maintaining links to Islamist insurgents in the country.