Gates, who arrived on Monday in Afghanistan, said he has not yet seen data on any jump in al-Qaeda activity, but he said rising levels of violence in the country are a concern and he plans to talk about it with other defence leaders from NATO nations operating in Kabul.

"I'm not worried about a backslide as much as I am (about) how we continue the momentum going forward," Gates said earlier. "One of the clear concerns that we all have is that in the last two or three years there has been a continuing increase in the overall level of violence."

The senior defence official -- discussing the matter on condition of anonymity due to security concerns -- said the US military is concerned and is looking for definitive signs of greater activity by al-Qaeda and foreign fighters, but the US has not seen enough proof to draw any final conclusions.

As Gates headed to Kabul, US officials also said they were considering the possibility of providing arms, training equipment and other support to Afghan tribes.

The effort would be modeled after those in Iraq to empower residents to police their own neighbourhoods.