"But there are other areas - large areas in the southern part of Afghanistan especially, but in parts of the east - where we are not winning," he said.
In these areas "more has to happen along multiple lines of operation in order for anybody by any metric to say that the Afghans are winning or the efforts of the coalition are winning," Gen McKiernan added.
US President Barack Obama authorised the deployment of up to 17,000 extra US troops to Afghanistan last month amid a major review of US policy in Afghanistan and Pakistan.
The US already has about 14,000 troops serving with the Nato-led mission.
There are also 19,000 US troops under sole US command charged with fighting Taleban and al-Qaeda insurgents.
Gen McKiernan said he was "very satisfied" with the contribution of British troops in Helmand.
"Our challenge in the southern part of Afghanistan is that we don't have enough of a persistent security presence in all the areas that allow the other lines of operation - better governance - to develop in that area," he said.
"So these US forces, this force uplift, of which the majority will be positioned in the southern part of Afghanistan, I think will change security conditions there this year."
Meanwhile, a roadside bomb in southern Afghanistan has killed one Canadian soldier and wounded four others.
The Canadian military said the soldiers were on patrol in Kandahar province when their vehicle was hit by the blast on Sunday.
More than 110 Canadian troops have been killed since the start of their mission in Afghanistan in 2002 - the third highest casualty figures among international forces there, after the US and UK.
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