U.S. Army General David McKiernan, the head of the NATO-led force in Afghanistan, said he needed three brigades plus support units -- possibly around 15,000 troops -- in addition to other forces scheduled to arrive in the coming months.
McKiernan's comments come amid growing Western concern about Afghanistan, where violence by Taliban militants and other insurgents has risen dramatically in the past two years.
Admiral Mike Mullen, chairman of the U.S. military's Joint Chiefs of Staff, said this month he was not convinced the United States was winning in Afghanistan.
"We're in a pretty tough fight here...and I think we're going to be here for a while, although I don't think the insurgency will ever win in Afghanistan," McKiernan told reporters traveling with Gates.
He said violence was up 30 percent this year compared to a year ago.
Some 33,000 U.S. personnel are among a total of nearly 71,000 foreign troops in Afghanistan, some led by NATO and some by a separate U.S. command.
Under plans announced by President George W. Bush last week, about 1,000 Marines are due to be deployed in Afghanistan in November to take over a mission training Afghan forces.
A U.S. Army brigade of around 4,000 soldiers is due to arrive in January but McKiernan said that unit was emergency assistance for U.S.-led forces facing resurgent militants in eastern Afghanistan, which borders Pakistan.
A request for three more brigades still stood in addition to those reinforcements, he said.
U.S. officials say Taliban militants have a safe haven in tribal areas of Pakistan and have been increasingly able to cross the border to mount attacks.
"Our fight in the east is different than we had predicted," McKiernan said. "It's a tougher fight."
U.S. forces have stepped up attacks against targets inside Pakistan in recent weeks with missile strikes from unmanned aircraft and a raid by helicopter-borne U.S. commandos.
Admiral Mullen traveled to Pakistan for talks with Prime Minister Yousuf Raza Gilani and military chief General Ashfaq Kayani. They were expected to discuss issues connected to the fight against militants in Pakistan, particularly on and around the Afghan border.
His visit follows a vow by Pakistani authorities to defend itself from cross-border raids against suspected militants by U.S.-led forces based in Afghanistan.
News that U.S. forces recently conducted a ground raid from the Afghan side of that border was followed by reports that said President Bush issued a secret directive in July that allows Special Forces to conduct ground operations in Pakistan.
Gates flew in from Iraq, where a "surge" of U.S. forces is widely credited with helping to reduce rampant violence.
But McKiernan said he was not asking for a surge as that implied only a short-term boost.
"I think what we need are...increased capabilities on a sustained basis, it's not a temporary basis," he said.
Gates will meet President Hamid Karzai, who has condemned the killing of civilians by foreign troops hunting insurgents.
McKiernan said he had issued an order earlier this month reminding NATO troops of rules for the use of lethal force in an effort to minimize civilian casualties.
But he said it was hard to avoid such casualties completely when facing an enemy that hid among the civilian population.
Gates will receive briefings on the military's use of air power to see if more steps can be taken to avoid civilian casualties, Pentagon press secretary Geoff Morrell said.
Nearly 1,500 Afghan civilians were killed in the first eight months of this year, many in attacks on schools, medical clinics, bazaars and other crowded areas, the United Nations said on September 16.
The death toll, up 39 percent from the same period in 2007, includes 800 killings blamed on Taliban and other militants as well as 577 caused by Afghan forces and their international allies. Responsibility for another 68 deaths was not clear.