"I think it's a real concern that the United States is having to bear a disproportionate part of the burden," he said.
Gates also said that his commander in the south of Afghanistan felt that Afghan forces and their international counterparts were "holding their own", but that this was "not good enough".
US troops are facing increasing
pressure from Taliban fighters [AFP]
US Army General David McKiernan, commander of Nato forces and most US troops in Afghanistan, had requested an extra US 20,000 troops, which would push US force levels in Afghanistan past 50,000.
Gates said he hopes to have three brigades of combat forces, about 10,500 troops, in Afghanistan by the middle of 2009.
The Pentagon has already scheduled the deployment one of the brigades in January that originally had been set to be sent to Iraq.
"Beyond January, we are hopeful we will be able to send an additional two brigade combat teams by late spring," Gates said.
"Until we get to this tipping point where there is sufficient security capabilities in Afghanistan, Afghan-led, there will probably continue to be a degree of insurgent violence," he said.
About 65,000 international troops are currently based in Afghanistan, including more than 30,000 from the United States.
Barack Obama, the US president-elect, has pledged to make Afghanistan one of his top priorities in his new administration and to send more troops there, asking Gates, a Republican, to remain as defence secretary for his tenure.
Seven years after US-led forces ended the rule by the Taliban following the September 11, 2001, attacks on the US, both Gates and McKiernan say sustained commitment to Afghanistan by the US and its allies was remain a priority for several years.
"The Soviets couldn't win in Afghanistan with 120,000 troops and they clearly didn't care about civilian casualties, so I think we have to think about the longer term in this," Gates said.