The United States would start withdrawing its thousands-strong troops from Afghanistan before 2013, well beyond what US President Barack Obama had promised.

According to Reuters, briefing a group of US lawmakers on his battlefield assessment, US top commander in Afghanistan General Stanley McChrystal, said that the drawdown would be attempted at "sometime before 2013".

The military official was being questioned on his suggestion of boosting the roughly 68,000-strong US contingent by 40,000 auxiliary servicemen. The US forces are accompanied by nearly 42,000 forces from US allied countries.

Meanwhile, Obama is expected to present a new strategic outline, during a nationwide address on Tuesday, which he claims would "finish the job" in Afghanistan.

The new strategy could involve committing as much as 35,000 more troops to the Afghanistan-based contingents.

Washington has also been pushing its Western allies to make generous troop contributions in an apparent attempt to satisfy the 40,000 benchmark.