"(The) problem with terrorism is they can always carry out a spectacular act that gains everyone's attention," Gates said during a briefing in Mexico City, where he met with several government leaders.

More likely, tactics are changing, he said, citing increased use of suicide and roadside bombings, as well as the assassination attempt Taliban leaders claimed they engineered.

"I think that we have seen a change in the nature of the Taliban threat, and the question is whether it's temporary or longer-lasting," Gates said. "And so I think that they're (Taliban) confronted with the firepower and the strength of the coalition forces in Afghanistan, that they are changing their tactics."

Reports of Taliban infiltration in Afghan security forces and the possible Iranian support, while important, don't signal a significant downturn in security in Afghanistan, he said.

Infiltration "has happened before and I expect it'll happen again," Gates said.

Regarding Iranian support, Gates said, "I do not have a sense at this point of a significant increase in Iranian support for the Taliban and others opposing the government in Afghanistan."