(PUL-E-CHARKHI, Afghanistan) -- U.S. Defense Secretary Robert Gates raised the possibility Wednesday that some of the U.S. forces involved in the Afghanistan surge could leave the country before President Barack Obama's announced July 2011 date to begin withdrawal.

Without giving specifics, Gates said, "It would have to be conditions-based."

Gates made the remarks during a visit to a dust-blown training ground in Kabul province where Afghan soldiers come for weeks of training under U.S. and British instruction. British Brigadier Simon Levy told Gates that if NATO countries contribute more trainers, the project to expand the Afghan army will keep pace.

The goal is to reach 134,000 trained forces this fall. The Pentagon hopes the Afghans will soon ease the load on U.S. forces.

In a press conference with Gates, Afghan Defense Minister Abdul Rahim Wardak said his troops are eager to take on the responsibility for defending the country, but gave no indication of when that might be possible.

Gates said, "We will begin that transition no later than July of 2011, but the pace will depend also on conditions on the ground."

Still, the Pentagon chief said, "We should not be too impatient."

Gates watched as Afghan troops dealt with a simulated roadside bomb explosion. He stood on an embankment above the road as Afghan soldiers leapt out of a convoy, tended to casualties and contained the explosive.

He said he was very impressed by what he saw.

"Although attention may be focused on operations in the south today, the training that is going on in this facility is even more important," he said. "At the end of the day, only Afghans will be able to provide long-term security for Afghanistan."

U.S. forces are engaged in a major offensive against Taliban fighters in southern Afghanistan. Gates visited some of those troops Tuesday.