Top US military officer Admiral Mike Mullen said on Tuesday more American troops were needed in Afghanistan as soon as possible to hold territory where insurgents have been routed.

Mullen told a news conference it was up to President Barack Obama to decide when to deploy additional troops to Afghanistan, but he said time was of the essence at what he called a critical period for the country.

During a visit to the Canadian capital to discuss the Afghan war among other issues, Mullen was asked about the possible reinforcement of the US mission as requested by the commander of US forces in Afghanistan, General David McKiernan.

"The general had this request out for many months and those working through the request recognise that the sooner the better with respect to this," Mullen said.

He said 2009 was a crucial moment with elections scheduled later in the year and an increasingly violent insurgency in the south and east of the country.

"I''m hopeful that we can get them there as soon as absolutely possible, but, again, that''s a decision for the president of the United States, not for me."

He said more US troops were needed to allow for development and aid projects to go ahead as insurgents were often moving back into areas where NATO forces had previously pushed them out.