Gen. David McKiernan said it's not just a question of troops but more economic aid and more political aid as well.
Speaking to Pentagon reporters, the head of NATO forces in Afghanistan said there has been a significant increase in foreign fighters coming in from neighboring Pakistan this year including Chechens, Uzbeks, Saudis and Europeans.
"The additional military capabilities that have been asked for are needed as quickly as possible," he said.
He said he was encouraged by recent Pakistani military operations against insurgents waging cross-border attacks into Afghanistan, but also said that it is too soon to tell how effective they have been.
Officials have said that violence in Afghanistan is up about 30 percent this year compared with 2007. The Taliban and associated militant groups like the terrorist network al Qaeda have steadily stepped up attacks in the last several years and more U.S. soldiers have died in Afghanistan already this year than in any year since the 2001 U.S.-led invasion.
"We're in a very tough fight," McKiernan said. "The idea that it might get worse before it gets better is certainly a possibility."
Defense Secretary Robert Gates said last week that he may be able to send thousands more combat troops to Afghanistan starting next spring.
McKiernan was scheduled to meet with President Bush at the White House late Wednesday.
The general's assessment coincides with a fresh report on the situation by U.N. Secretary-General Ban Ki-moon, who expressed dismay that attacks against aid workers have increased in 2008.
A report released by his office Tuesday said that at least 30 aid workers have been killed and 92 abducted so far this year. At least 22 World Food Program convoys have been attacked, as have 59 schools.
"Regardless of the progress made in certain areas, my overall impression is that the situation in the country has deteriorated over the past six months," Ban said in the report. "Nevertheless, I strongly believe that the negative trend can be reversed."
Saying that security has "deteriorated markedly," the report noted that the number of U.N.-recorded security-related incidents rose to 983 in August the highest monthly total since the Taliban's ouster in late 2001. The report did not define a security incident, but typically it refers to bombings, shootings and other violent acts.