The gradual Taliban resurgence in the south, which has accelerated this year, marks the biggest setback to the United States and its allies since the Afghan war began in 2001, Afghan analysts say.

The Taliban demonstrated their growing power in the south Thursday by blowing up a bus carrying Afghan workers from Kandahar city to the main coalition base there, killing at least eight. The morning rush-hour bombing marked the first major attack targeting Afghan coalition employees -- apparently a new Taliban tactic.

After the bloodiest springtime in the war, with 400 people killed in May, the summer could see a broad, critical battle for control of the south, just as the Bush administration is trying to hand off a significant combat role to NATO and bring some U.S. forces home before November congressional elections. NATO is due to take command of combat forces in the southern provinces by August.

The Taliban are trying to knock NATO off balance from the start, guerrilla commanders have told journalists in neighboring Pakistan, where many Taliban are said to be based.

More than 11,000 Western and Afghan troops plan to sweep through at least four southern provinces "to extend the reach of the government of Afghanistan into areas that up to now have really been under Taliban influence, if not Taliban control," British Col. Chris Vernon, the chief of staff for coalition forces in southern Afghanistan, told reporters. Operation Mountain Thrust, as the offensive is named, will last "as long as it takes, but we're certainly looking at about a month," Vernon said.

The offensive follows Operation Mountain Lion, a smaller assault in the northeastern province of Kunar that also was meant to eliminate Taliban, al Qaeda and allied fighters. The U.S. command claimed success in that offensive, but another U.S. official and Afghan analysts said it yielded only a few arms caches and a handful of militant casualties.

A key battleground in the south will be densely populated farming areas outside Kandahar, Afghanistan's second-largest city, which served as the political headquarters of the Taliban when they ruled Afghanistan from 1996 to 2001.

The offensive is to center in Helmand, Kandahar, Zabul and Uruzgan provinces, said Vernon.

This article appeared on page A - 14 of the San Francisco Chronicle