Investigators inspect a bomb blast site in Karachi on January 30, 2013. – Photo by AFP

KARACHI: A bomb blast killed three suspected militants in Karachi on Wednesday as they were carrying out preparations for a deadly attack, police said.

The suspected militants were attempting to put the bomb in the trunk of a taxi when it went off, senior police officer Aleem Jafri told AP. The device exploded accidentally.

The men had prepared three other bombs, but they did not explode and were defused by police.

The bombs, which were wired with electronic detonators, contained about 300 kilograms (660 pounds) of explosives, Jafri said.

Safdar Ali, a local police official told had told AFP the bomb exploded minutes after a hand grenade was hurled at a police van, a kilometer away.

“Around 30 minutes before the bomb explosion, two persons came on a motorbike and hurled a grenade on a police van. Three policemen and one civilian were injured in the explosion caused by the grenade,” he said.

“We are gathering details of both events. We suspect that the two people killed in the second incident may be the terrorists trying to plant the bomb,” Abdul Kareem Mughal, a senior police official, had told AFP.

Meanwhile, Governor Sindh Dr Ishratul Ibad took notice of the blast shortly after media reports. DawnNews reported that the governor had directed the DIG West of Police to submit a report regarding the blast.

Karachi has a long history of violence carried out by Islamic militants and gangs associated with the city's main political parties.