Hillary Clinton had outside advisers advising her on Afghanistan, personal emails show

In the autumn of 2009, as US President Barack Obama conducted a long, divisive review of whether to pour more troops into Afghanistan, an influential group of advisers were quietly pushing a hawkish line.

The advisers didn't work for Obama's White House, however. They were veterans of President Bill Clinton's administration and they peppered Obama's secretary of state, Hillary Clinton, with messages urging a robust counter-insurgency effort in Afghanistan and a tougher US stance towards Pakistan, according to emails released by the State Department.

The emails reveal how, even as Obama ran a highly formalised Afghan policy review of near-endless meetings and position papers, Clinton was receptive to outsiders' views delivered through back-channels.

How much they influenced Clinton, who was also getting plenty of advice on Afghanistan and Pakistan from officials at her State Department, remains unclear. But Clinton eventually threw her support behind a troop "surge" and there is some evidence the external advisers formed part of her thinking.

Some had more national security expertise than others, but all appeared to have Clinton's ear - and her private email address.

In one missive on October 11, 2009, retired General Wesley Clark, a confidant of the Clintons, warned against repeating in Afghanistan the "incrementalism" of gradual troop increases during the Vietnam War.

"Hopefully, we can be more decisive: lean harder on the Pakistanis, provide more troops to [Afghanistan commander General Stanley] McChrystal ... and raise the heat on al-Qaeda," Clark wrote.