Robert Gates, US defence secretary, on Tuesday stressed that it was “impossible” to determine when US troops would return home from Afghanistan, underscoring the increasingly tough security situation in the war-torn country.

Barack Obama recently approved the deployment of another 17,000 US troops to Afghanistan as he shifts the focus of the US military from Iraq to the central Asian country amid increasingly brazen attacks by the Taliban and al-Qaeda militants. Currently, about 38,000 US troops are stationed in Afghanistan.