John Kerry makes surprise visit to Afghanistan

US Secretary John Kerry made an unannounced trip to Afghanistan on Saturday to talk with rival leaders and to help ease a political crisis in the country.

Kerry made the visit at a tough time for Afghanistan as the country's government is rocked by inner conflicts between political rivals, a declining economy and the strongest resurgence of the Taliban since they were ousted in 2001.

US State Department spokesman John Kirby said Kerry is to meet Afghan President Ashraf Ghani, the victor of Afghanistan's disputed 2014 election, and Chief Executive Abdullah Abdullah, the runner-up, and will "emphasise US support" for Afghanistan and its defence forces.

US forces in Afghanistan are set to be almost halved from 9,800 to 5,500 at the beginning of 2017 and General John Nicholson, the new top US commander in Afghanistan, is running a review of security.

The political deal that Kerry brokered suggested that their power-sharing arrangement would end in September 2016, which has fueled political maneuvering in Kabul. An opposition movement close to former president Hamid Karzai is pushing to have a say on the way forward.

The legal decree that enacted the deal, however, provided no clearly binding time limit, leaving open the possibility that the National Unity government, in one form or another, could continue on for the rest of Ghani's five-year mandate.

Kerry is expected to make that point while in Afghanistan, a stance that US officials hope may help quell some of the infighting.

"Though the political agreement calls for this to be a two-year agreement, the decree doesn't spell out an end date," said a senior US official on condition of anonymity. "We ourselves here don't view that there is going to be an end ... in September."

Richard Olsen, US special representative for Afghanistan and Pakistan made a similar point in Washington on Monday.

"The Secretary wants to signal continuing US support for the national unity government," he said. "It's at the 18-month mark in a five-year term."

In July, the NATO Western security alliance is expected to decide how to fund Afghanistan's security forces in the coming years and donor nations will gather in Brussels in October to make civilian aid pledges to Afghanistan.