US President Barack Obama has said the Pakistan-Afghanistan border remains the epicentre of al Qaeda activities. While acknowledging that al Qaeda's branch in Yemen has become a more serious problem, President Obama ruled out sending troops to Yemen at this point of time.In an interview in Washington on Sunday, Mr. Obama said he believes the centre of Al-Qaeda activity was still along the border between Afghanistan and Pakistan. US has currently deployed a large number of troops in Iraq and Afghanistan.Al Qaeda's activities in Yemen came to prominence following the failed attempt to bomb a US plane by a Nigerian national, whose responsibility was later claimed by al Qaeda in Arabian Peninsula, which has its base in Yemen.