Abuja, July 7 (IANS): At least 20 people were killed in a bomb explosion that rocked Zaria city in Nigeria's northwestern state of Kaduna on Tuesday, provincial governor Nasir El-Rufai said.

The explosion occurred in Sabon Gari area of Zaria city, at a centre opened for biometric screening of state civil servants, the governor said.

"I am sad that a terrorist bomb attack just killed 20 people in Zaria. May the souls of our innocent departed rest in peace," the governor said in a statement.

Witnesses said many people were feared injured in the attack, according to Xinhua news agency. It is unclear if the attack was carried out by a suicide bomber.

"We call on our citizens to be vigilant and avoid crowded places like markets, mosques, churches and motor parks in the next few weeks. We call on our people to report any suspicious persons and movements to law enforcement agents, traditional rulers and religious leaders," the governor said, adding the government was working with federal security agencies that were taking proactive measures to frustrate terrorists.