Maj. Gen. Jeffery J. Schloesser said a 40 percent surge in violence in April and May was fueled in part by militants preparing stores of weapons during the winter, which generally is a slow period for fighting, particularly in snowy Afghan mountainous areas.
"If we don't do anything over the winter the enemy will more and more try to seek safe haven in Afghanistan rather than going back to Pakistan," Schloesser said.
U.S. and NATO officials say militants cross into Afghanistan from Pakistan, where they rest, train and resupply in tribal areas along the frontier where the Pakistani government has little sway.
Schloesser estimated 7,000 to 11,000 insurgents operate in the eastern part of Afghanistan that he oversees - a far higher estimate than given by previous U.S. commanders.
He said the U.S. military realized more militants spent last winter in Afghanistan after speaking with elders and villagers who had been pushed out of their homes. The spike in violence in the spring occurred because insurgents were already in position to unleash attacks, though U.S. officials didn't know it at the time, he said.
"They didn't have to come over the passes, they were already here," Schloesser said during an interview while flying in a Black Hawk helicopter Monday to a small U.S. outpost in Nuristan, a province that borders Pakistan.
A NATO spokeswoman said she didn't believe increased operations would take place over the winter in other areas of Afghanistan where the U.S. isn't the primary military force.
Attacks in the eastern part of Afghanistan where U.S. troops primarily operate were 20 percent to 30 percent higher in June and July than a year earlier, Schloesser said.
He said an attack by six or so suicide bombers on a large U.S. base near the Pakistan border Aug. 18 was carried out by Arabs and Chechens, foreign militants who are increasingly flowing into the Afghan theater. He said militant Web sites have been encouraging fighters to go to Afghanistan instead of Iraq.
"I can't prove they are coming from Iraq to Afghanistan, but I've seen it on Web sites that that's what they're being told to do," Schloesser said.
This year is on pace to be the deadliest for U.S. and NATO troops in Afghanistan since the invasion that ousted a Taliban regime in late 2001, as militants increase the complexity and scale of their assaults. Nearly 200 soldiers in the international forces have died this year, including 105 Americans. The total for all of last year was a record 222.
On Wednesday, the Canadian military said three of its soldiers were killed and five wounded when their patrol was attacked in the volatile Zhari district near Kandahar city in the south.
U.S. and NATO commanders have been urging that U.S. troop levels in Afghanistan be increased by about 10,000. There are already 34,000 American troops in Afghanistan, the highest since the war began. That includes 15,000 in a NATO force of 65,000, also a high for the war.
Schloesser said he was "reasonably optimistic" that he would see the additional American troops in the next several months. He said leaders in Washington "understand the importance of what our people are doing here."
In the last two months, troops in the U.S.-led coalition have killed six top insurgent leaders in a valley 40 miles northeast of Kapisa, Schloesser said. The top militant leader in the Tagab Valley of Kapisa province is still on the loose.
U.S. and NATO leaders keep a wary eye on Tagab because it is close to Afghanistan's capital, Kabul, and the big U.S. military base at Bagram.
"I still want to get (militant leader) No. 1. I'm waiting for him to come back into the country," Schloesser said. "I don't believe he's in the valley right now, otherwise he'd be captured or killed."