Small bombs exploded at 18 Shell petrol stations in the Pakistani city of Karachi on Thursday, three days after suicide bombings on Western targets in oil-rich Saudi Arabia killed 34 people.

Shell Pakistan said one customer, three station attendants and one security guard received minor injuries in the attacks, which began before dawn and hit a total of 18 stations.

Police said there were no serious injuries or damage.

Tariq Jameel, a deputy inspector general of police in the port city, said small devices packed into boxes were placed inside garbage bins at the stations and appeared aimed to scare. "The aim was to create panic and the target was Shell," he said.

Pakistan's main commercial city has seen a series of attacks against Western targets blamed on Islamic extremists in the past year. Police have rounded up suspects in recent months, including some suspected members of Al Qaeda.

Shell said it was "shocked and concerned" by the attacks and had taken steps to tighten security at all its facilities in Pakistan. Jameel said security had been tightened at all foreign business interests in Karachi.

Kashif Jalal, an official of the police Bomb Disposal Squad, said the explosions were caused by time bombs, each weighing about 200 grams.

Sajjad, an employee at one of the stations, said a rubbish bin blew up with a large bang, smashing the glass of his kiosk. "My ears were numb for a couple of minutes," he said.

Shell Pakistan, a subsidiary of Anglo Dutch giant Royal Dutch/Shell Group, is one of Pakistan's biggest foreign-invested firms.

Past attacks in Karachi have been blamed on extremists enraged by Pakistan's decision to back the US-led war on terror, launched after the September 11 attacks in 2001.

The violence has hurt Pakistan's efforts to boost depressed foreign investment.