QUETTA, Pakistan (Reuters) - A bomb blast killed three soldiers and two civilians in the Pakistani city of Quetta on Wednesday, security officials said.

The bomb, attached to a motorcycle, exploded near a security vehicle escorting school children, they said. Sixteen people were wounded. A Reuters reporter saw the corpses of three soldiers under the vehicle as it burned.

There was no immediate claim of responsibility for the attack in Baluchistan, Pakistan's biggest but poorest province which borders Afghanistan and Iran, and where several militant groups are active.

Ethnic Baluch separatists are waging an insurgency, demanding more autonomy and control over natural resources. Pro-Taliban militants also operate in the area and Sunni Muslim militants regularly carry out attacks on Baluchistan's Shi'ite Muslim minority.

In a separate attack, suspected militants shot dead four policemen in the northwestern town of Bannu, a police official said. Pakistan's Taliban movement claimed responsibility.

(Reporting by Saleem Shahid; Writing by Michael Georgy; Editing by Mark Heinrich)