The Taliban have gained the upper hand in Afghanistan, the top American commander there said, forcing the U.S. to change its strategy in the eight-year-old conflict by increasing the number of troops in heavily populated areas like the volatile southern city of Kandahar, the insurgency's spiritual home.

Gen. Stanley McChrystal warned that means U.S. casualties, already running at record levels, will remain high for months to come.

In an interview with The Wall Street Journal, the commander offered a preview of the strategic assessment he is to deliver to Washington later this month, saying the troop shifts are designed to better protect Afghan civilians from rising levels of Taliban violence and intimidation. The coming redeployments are the clearest manifestation to date of Gen. McChrystal's strategy for Afghanistan, which puts a premium on safeguarding the Afghan population rather than hunting down militants.

Gen. McChrystal said the Taliban are moving beyond their traditional strongholds in southern Afghanistan to threaten formerly stable areas in the north and west.

The militants are mounting sophisticated attacks that combine roadside bombs with ambushes by small teams of heavily armed militants, causing significant numbers of U.S. fatalities, he said. July was the bloodiest month of the war for American and British forces, and 12 more American troops have already been killed in August.

"It's a very aggressive enemy right now," Gen. McChrystal said in the interview Saturday at his office in a fortified NATO compound in Kabul. "We've got to stop their momentum, stop their initiative. It's hard work."

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