PESHAWAR, Pakistan (REUTERS) - At least fifteen people were killed when a bomb exploded on a bus carrying government employees in the volatile north-west Pakistani frontier city of Peshawar on Wednesday, officials said.

The attack took place on a main road after the bus picked up government workers from districts surrounding Peshawar and was transporting them to work in the city.

Senior police official Mohammad Kashif said 15 people were killed and 35 wounded. A hospital official said the death toll was likely to rise.

The attack took place in an area where Pakistani security forces have stepped up their fight against the Taleban and other militant groups along the border with Afghanistan. This followed the massacre of 134 children at an army-run school in Peshawar in December 2014.

Bomb disposal squad officials said 10 kg of explosives were planted on the bus and further investigations were underway.

"It's premature to comment about the nature of the blast but it appears that explosives were planted inside the bus," Mr Kashif said. "There were 40-50 people on the bus."

The 35 wounded were taken to Peshawar's main Lady Reading Hospital, hospital spokesman Jamil Shah said.

"Emergency has been declared and all doctors have been called in to handle trauma," he said.

Many of the wounded were in critical condition and the death toll could rise, he said.

Attacks have fallen since the government crackdown after the 2014 school attack and the Taliban have been squeezed into small pockets of territory. However, militant groups remain able to launch sporadic attacks on security forces and civilian targets.

Two Pakistani employees of the US consulate in Peshawar and some soldiers were killed by a bomb while on a drug-eradication mission earlier this month.