John Kerry arrives in Kabul on unannounced visit

US Secretary of State John Kerry arrived on an unannounced visit to Kabul on Friday amid troubled negotiations with Afghanistan on US troops staying in the country after 2014.

President Hamid Karzai said this week that he was prepared to walk away from the Bilateral Security Agreement (BSA) talks if Afghanistan was not happy with its conditions.

But the United States has repeatedly pushed for the pact to be signed by the end of this month so that the US-led NATO military coalition can schedule its withdrawal of 87,000 combat troops by December 2014.

"President Obama and President Karzai reaffirmed both back in January that the goal here was to complete the BSA in October," a state department official travelling with Kerry told reporters.

"We continue to believe that is both preferable and doable. It is only October 11 at this point and obviously it is desirable.

"Uncertainty about an incomplete BSA could erode the resolve among NATO allies, makes (it) more difficult to plan for the US, makes (it) more difficult to plan for our NATO allies. The goal has not changed and that is what we continue to press for."

But Karzai said he refused to be rushed into signing the deal, and would first seek approval from a traditional grand assembly to be convened in about month's time.