"We know the Taliban have been more active in recent weeks and months and there are indications that they may be planning even larger attacks, more spectacular attacks," said Zalmay Khalilzad. "Our forces and our coalition partners are prepared to prevent and respond to any increased Taliban activity."
He called on neighboring Pakistan to do "a lot more" to clamp down on Taliban insurgents and terrorists sheltering on its side of the border.
"Pakistan cannot become a sanctuary for the Taliban and al Qaeda people who want to attack Afghanistan," Khalilzad said at a news conference for Afghan journalists.
Last Thursday, Pakistan's army swooped down on a suspected al Qaeda mountain hideout in the country's northwest in its largest-ever offensive against Osama bin Laden's network. Eight suspected terrorists were killed and 18 others captured.
But Afghan and Western officials have long complained that the country's border regions have become a safe haven for militants, who cross back and forth across the porous frontier to launch attacks.
Khalilzad said the Taliban see the rebuilding of war-shattered Afghanistan as a threat, and that was the main reason for the recent spate of attacks against aid workers and government officials. Taliban rebels and al Qaeda operatives have also launched increasingly bolder assaults on coalition troops.
"The Taliban see the building of roads and schools as a weapon against themselves," he told a meeting of Afghan journalists. "This indicates the kind of people they are and what they want for Afghanistan. They want to take it back. We will do everything we can to prevent them from succeeding."
Khalilzad, who has been nominated by U.S. President George W. Bush to be America's next ambassador to Afghanistan, is in the country for about 10 days to meet President Hamid Karzai and other top officials to discuss the latest security and political situation.
There are still 11,500 U.S.-led coalition troops in the country, hunting down Taliban and al Qaeda militants holdouts who appear to have regrouped.
There is also a 5,000-strong, NATO-led peacekeeping force, but it is restricted to Kabul. Karzai's government wants the force to deploy to other parts of the country where warlords and their private militias are largely in control.
NATO leaders in Brussels are reviewing plans that could involve 2,000 to 10,000 more peacekeepers fanning out to major provincial cities.
Attacks by Taliban militants and bandits have led to the suspension of aid projects in many parts of the country.
There were no ceremonies in Afghanistan to mark the anniversary of the Oct. 7, 2001, start of Operation Enduring Freedom, which ousted the Taliban two months later. Sixteen American troops died and uncounted numbers of Taliban and civilians lost their lives during the war itself, and 20 U.S. soldiers have been killed in fighting since then.
In the latest of nearly daily attacks against American forces, suspected Taliban rebels fired rockets at two U.S. bases in southern Afghanistan on Monday and Tuesday, the U.S. military said. There were no casualties.
Meanwhile, the first foreign bank opened for business in Afghanistan on Tuesday in a move officials hope will persuade people to bring back billions of dollars from abroad to boost reconstruction.
Afghanistan's banking system collapsed along with its communist government in 1992, when the Islamic mujahedeen captured Kabul. Afghans scrambled to move their money abroad, often to Pakistan.
The National Bank of Pakistan branch opened in a heavily guarded house near the center of the city.