KARACHI (AFP) - At least two people were killed and seven others wounded after a bomb planted on a motorcycle exploded outside a mosque in Karachi on Friday, police said.

The blast took place in the congested Aram Bagh area of Pakistan's biggest city, outside a mosque belonging to a sub-sect of the minority Shi'ite Muslim community.

Pakistan has been hit by a rising wave of sectarian violence in recent years, most of it carried out by militant Sunni Muslim groups targeting Shi'ites.

A police spokesman said two were killed in the blast and seven wounded.

Saif-ud-Din, a witness, said people were coming out of the mosque after Friday prayers when there was a huge blast and many people fell to the ground.

"I was inside the mosque when the bomb exploded and I saw people falling to the ground," Din told AFP.

Umer Khatab, a senior officer at the police Counter Terrorism Department, told reporters that about 2kg of explosives were used in the bomb, which was detonated with a timer.

"The bomb was planted to target the people coming out of the mosque," Khatab said.

Many worshippers in bloodstained clothes gathered outside the mosque, searching for relatives.

The blast ripped through the shutters of several nearby shops and shattered windows.

In a separate incident a suicide bomber on Friday evening rode a motorbike into a vehicle carrying paramilitary troops in the east of the city killing two paramilitary soldiers.

"A suicide bomber riding on a motorbike hit the mobile van of (paramilitary) rangers in the neighbourhood of North Nazimabad killing two soldiers," senior police official Faisal Noor told AFP.

He said three other soldiers in the vehicle were injured and one was in critical condition.

In January, 61 people were killed in a suicide bombing carried out by a Taleban splinter group at a Shi'ite mosque, the deadliest sectarian incident to hit the country since 2013.

Karachi, a city of 18 million, is rife with criminal, ethnic, political and sectarian killings, which claim hundreds of lives each year.