(Adds details, Gates quotes)
By Saeed Ali Achakzai
SPIN BOLDAK, Afghanistan, Feb 12 (Reuters) - At least 700 Taliban fighters have crossed from Pakistan into Afghanistan to reinforce guerrillas attacking a key dam, a major source of electricity and irrigation, a provincial governor said on Monday.
"We have got confirmed reports that they are Pakistani, Uzbek and Chechen nationals and have sneaked in," Helmand Governor Asadullah Wafa told Reuters by telephone.
The Kajaki dam has seen major fighting in recent weeks between the Taliban and NATO forces, mainly British and Dutch.
NATO-led troops have been conducting operations in the area for several months to allow reconstruction on the dam and the power transmission lines to boost output, after fighting halted refurbishment last year.
A spokesman for the NATO-led force confirmed rebel movements in the dam area, including across the border, but could not confirm the governor’s numbers or any other details.
Built by the Soviets in 1953, the dam irrigates about 142,000 hectares (285,000 acres) of farmland and two hydroelectic plants built by the United States in 1975 have a capacity of 33 megawatts. A third plant is planned, which would almost double that capacity by 2009.
NATO, U.S. and Taliban commanders warn a major offensive will come in spring when the snows melt in a few months, after the bloodiest year since the hardline Islamists were ousted by a U.S.-led coalition in 2001.
PAKISTAN BACKS TALIBAN - GOVERNOR
More than 4,000 people died in fighting last year.
Wafa said the Taliban fighters were brought in by local commanders for a joint operation with al Qaeda.
"They are planning to destroy the Kajaki dam," he said, accusing Pakistan’s military intelligence agency, ISI, of providing training and logistical support for the guerrillas.
"Pakistan is supporting the Taliban in order for them to keep fighting on in Afghanistan. They don’t want Afghanistan’s development and reconstruction," he said.
Pakistan denies continuing to support the Taliban, its former protege, but Afghan officials say it still does. The United States says the guerrillas benefit from safe havens in Pakistan and support from fellow Pashtun tribes, but also rejects charges Islamabad officially supports the resurgent rebels.
Wafa’s comments came as U.S. Defense Secretary Robert Gates visited Pakistan and met President Pervez Musharraf briefly on his way home from NATO talks in Spain in which Afghanistan and calls for more foreign troops features prominently.
"We talked about the importance of seizing the offensive this spring to deal the Taliban and al Qaeda a strategic set-back," Gates told reporters afterwards.
He also said the United States would not repeat the mistake of letting extremists take control of Afghanistan.
"After the Soviets left the United States made a mistake. We neglected Afghanistan and extremism took control of that country," Gates told a news conference at the Chaklala military air base in the garrison city of Rawalpindi.
"The United States paid a price for that on Sept 11, 2001. We won’t make that mistake again."
Several Taliban fighters were also killed on Monday in an attack targeting a senior guerrilla leader closely linked to the Islamist movement’s fugitive leader, Mullah Mohammad Omar, the U.S.-led coalition said in a statement.
It did not name the leader, nor say he was caught in the operation by U.S.-led troops.
The militants were killed near the town of Greshk in Helmand, a Taliban bastion and the main drug producing region of Afghanistan, the world’s largest producer of heroin. (Additional reporting by Kristin Roberts in Rawalpindi and Sayed Salhuddin in Kabul)