Kerry aims to ease crisis over Afghan unity pact during Kabul visit

Kerry's unannounced visit to Kabul was intended to demonstrate support for the national unity government headed by President Ashraf Ghani, victor of the disputed election of 2014 and his runner up Chief Executive Abdullah Abdullah.

The visit comes at a difficult moment for Afghanistan, with Ghani's government weakened by infighting between rivals, the economy sinking and the resurgent Taliban stronger than at any time since they were toppled from power in late 2001.

Underlining the precarious security situation, at least two explosions, apparently from rockets, hit the diplomatic zone of the capital shortly after Kerry's visit ended and his plane was preparing to take off from Bagram airport outside the city.

Kerry repeated an offer of peace talks with the Taliban and called on Afghanistan's fractious politicians to work together.

"Democracy requires credible institutions," he said. "Even more than that, it requires a willingness of people … from different political and ethnic and geographic factions to be able to come together and work for a common good."

Under the national unity deal, a loya jirga, or special assembly, was expected to be held to amend the constitution within two years of the September 2014 election, potentially allowing Abdullah to take the post of prime minister.

The approach of September's end of the two-year period has helped fuel growing political tension, with opposition politicians close to former President Hamid Karzai insisting that the unity agreement must be subject to broad consultation.

However Kerry said the agreement was valid for the full term of the government, suggesting that the US believes it can continue even without a new constitutional deal.

"Let me make this very, very clear because I brokered the agreement … There is no end to this agreement at the end of two years, or in six months from now," Kerry told a joint news conference with Afghan President Ashraf Ghani.

"This is an agreement for a unity government the duration of which is five years," he said.