KABUL, Afghanistan — Even after an extra U.S. Army brigade joins the fight against the insurgency here in January, three times that many reinforcements will be needed shortly thereafter, the highest-ranking U.S. general here said Tuesday.

Gen. David McKiernan, commander of NATO-led international forces in Afghanistan, told reporters traveling with Defense Secretary Robert Gates that the brigade arriving in January is an urgent requirement based on an assessment that fighting in eastern Afghanistan is tougher than believed six months ago.

"There are an additional three brigade combat teams" that have been validated by the Pentagon as a requirement, McKiernan said. He would not say exactly how many extra soldiers that entails, but said that it was more than 10,000 — beyond the roughly 3,700 in reinforcements that are scheduled to arrive in January.

There currently are about 33,000 U.S. troops in Afghanistan and about 146,000 in Iraq.

Gates arrived in the Afghan capital Tuesday evening after presiding at a ceremony in Baghdad where Gen. Ray Odierno took over for Gen. David Petraeus as the top U.S. commander in Iraq. Gates was meeting over dinner in Kabul with McKiernan and was to hold talks with senior Afghan officials on Wednesday.

More U.S. forces have been killed in Afghanistan so far this year than in all of 2007 as a resurgent Taliban-led insurgency has adopted bolder and often deadlier tactics. U.S. officials say the insurgency cannot win a conventional war, but its persistence has left U.S. and NATO leaders seeking reinforcements and has eroded the credibility of Afghanistan's fragile elected government.

McKiernan said he had no doubt that the insurgency could not win in Afghanistan, but he did not say U.S. forces are assured of victory, either.

"We are not losing, but we are winning slower in some places than others," he said.

In the interview, McKiernan also disclosed that he recently issued a revised order meant to govern the tactics and procedures followed by U.S. forces when engaging in air and ground fights against the insurgents. The revision, issued Sept. 2, was in response to a series of attacks that resulted in civilian deaths — most notably the highly publicized allegations that a U.S. attack on an Afghan village compound on Aug. 22 killed as many as 90 Afghan civilians, including women and children. The U.S. military has disputed the allegation but also has launched a new investigation in light of emerging evidence.

McKiernan said 90 percent of his new directive is meant to re-emphasize existing procedures.

"We've put an increased focus on partnering with Afghan security forces," he said in explaining the main change. "In other words, we want to run more and more operations that are combined operations with the Afghan army and/or the Afghan police. That's probably a new emphasis on this tactical directive."