A suicide bomber on a motorbike killed 13 people and injured 36 in a crowded square in southwest Afghanistan's Farah early on Friday.

Farah Governor Rohul Amin said more than a dozen of the 36 injured in the blast were in a critical condition, and "the death toll may rise" further, Al Jazeera reported. Earlier reports said six people were killed.

A doctor at the hospital in Farah told the channel that many of the wounded were children.

News agencies earlier quoted Farah Province Police Chief Faqir Mohammad Askar as saying the blast went off in an area where heavy goods vehicles were being loaded.

Farah, a mainly desert province along the border with Iran, is one of the areas that has seen a rise in attacks this year as Taliban fighters have spread to the west and north from their traditional bases in the south and east.

The attack comes a day after Afghan President Hamid Karzai was sworn in for a second term.