Vice President Joe Biden, in a somber assessment of the road ahead, predicted Sunday that U.S. casualties would climb in Afghanistan as the Obama administration shifts military priorities in the battle against terrorism.

"We've inherited a real mess" in Afghanistan, Biden said. "We're about to go in and try to essentially reclaim territory that's been effectively lost. ... All of this means we're going to be engaging the enemy more now."

One of President Barack Obama's first major foreign policy challenges is to confront an increasingly aggressive Taliban by trimming U.S. forces in Iraq and bolstering the troop commitment in Afghanistan.

But the complexity and potential cost of the new strategy were underscored Sunday by an outcry from Afghanistan over a U.S. operation that the United States said killed 15 militants but Afghan officials said had claimed the lives of 16 civilians, including two women and three children.

In Kabul, President Hamid Karzai condemned the strike, saying that repeated American military operations in which civilians are killed are "strengthening the terrorists."

Beyond the latest incident, the situation in Afghanistan reflects an earlier decision by the Bush administration and its allies to limit military involvement there — an approach that has opened the way for a resurgent Taliban that now rules unchallenged in much of the countryside and stages effective hit-and-run attacks on the urban areas where U.S. and other forces are concentrated.

And the Taliban's continued ability to operate from bases and staging areas across the border in northern Pakistan, with relatively little opposition from a weakened Pakistani government, adds to the problem for U.S. strategists.

Obama has pledged to deploy some 20,000 additional troops in Afghanistan in an Iraq-like "surge" designed to impose security in cities and towns that essentially have gone lawless.

That is a major increase, but the current force numbers only about 32,000 — far smaller than the roughly 200,000 serving in Iraq and only a fraction of what experts say would be needed to dominate the region.

Add to this Afghanistan's history of bloody but successful resistance to outsiders. Remote, mountainous and riven by tribal loyalties and a network of local warlords with shifting alliances, Afghanistan has been a graveyard for foreign military forces, including the Soviet Union and imperial Britain.

It was against this grim background that Biden, asked whether Obama's surge in Afghanistan would lead to more American casualties, said: "I hate to say it, but yes I think there will be. There will be an uptick."

The vice president did not provide details of how the additional forces would be used, beyond saying they would help train Afghan police and try to reclaim land. He did not say how many forces, for example, might be sent to the border with Pakistan, where many militants move easily across the rugged terrain and where al-Qaida leaders are believed to be hiding.

Biden, who traveled to Pakistan shortly before being sworn in as vice president, declined to comment on reports that a U.S. drone crossed into that country last week and attacked an al-Qaida post — but he reiterated Obama's statements during the campaign that he would not hesitate to strike within Pakistan if there was "actionable intelligence."

Gen. David H. Petraeus, head of Central Command, is conducting a review of the military situation in Afghanistan that is expected to be completed within weeks.