Several explosions hit Afghanistan's capital Kabul Saturday night shortly after US Secretary of State John Kerry exited the city. Kerry was in Kabul on a surprise visit in a bid to urge the Taliban to re-enter peace talks.

According to reports, authorities did not mention the number of casualties or what the nature of the blast was.

"We heard several explosions, but we don't know what caused them," an Afghan police official was quoted as saying by Agence France-Presse.

Kerry's surprise visit is being seen as a support to Afghan President Ashraf Ghani, who heads the national unity government, the Guardian reports. Kerry suggested the coalition government in Afghanistan should continue beyond September. He also made an offer of conducting peace talks with the Taliban and urged Afghanistan's politicians to work together.

"Democracy requires credible institutions. 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," Reuters quoted Kerry as saying in a statement to the media.

Afghanistan is currently facing a difficult time. The Ghani government has been weakened due to disharmony in the rival groups within the government and the Taliban has become stronger. The Afghan economy is also "sinking".

The Taliban have reportedly refused to participate in the peace until their conditions are met, which includes demand of withdrawal of about 13,000 foreign troops from Afghanistan.

A special assembly is expected to be held to amend the constitution within two years of the September 2014 election, which would allow Abdullah Abdullah to take up the post of a prime minister. Opposition leaders (close to ex-president Hamid Karzai) insist the agreement must be in consultation with them.

"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. This is an agreement for a unity government, the duration of which is five years," Kerry was quoted by the Guardian as saying during a joint news conference with Ghani.

Kerry then flew out of Kabul for the rest of the tour, which included Iraq and Bahrain so far, the AFP report said.