The top US military officer said Saturday that the Pentagon could double the number of American forces in Afghanistan by next summer to 60,000 - the largest estimate of potential reinforcements ever publicly suggested. Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said between 20,000 and 30,000 additional US troops could be sent to Afghanistan to bolster the 31,000 already there. This year has been the deadliest for US forces in Afghanistan since the 2001 invasion to oust the Taliban for hosting al-Qaida leader Osama bin Laden. Suicide attacks and roadside bombs have become more dangerous, and Taliban fighters have infiltrated wide swaths of countryside and now roam in provinces on Kabul's doorstep. US commanders have long requested an additional 20,000 troops to aid Canadian and British forces in two provinces just outside Kabul and in the south. But the high end of Mullen's range is the largest number any top US military official has said could be sent to Afghanistan.