A series of explosions shook 18 Shell gas stations in the southern Pakistani city of Karachi (searchearly Thursday, slightly injuring four employees, police and company officials said.

Police spokesman Malik Sheikh said two men on a motorcycle went from one station to the next before dawn, placing small explosive devices in garbage cans.

"They were are all Shell pumps that were targeted," Police Chief Kamal Shah said, adding that additional security had been deployed to several foreign-based businesses.

No one claimed responsibility for the attacks, but law enforcement officials who have raided militant organizations have seized maps of Karachi with Shell stations marked as possible targets.

Royal Dutch-Shell Group is a British-Dutch firm based in London -- but like many Western-owned businesses in Pakistan (search), it is frequently mistaken for a U.S. company.

Police said one of the explosions, in Karachi's posh Clifton neighborhood, was caused by a homemade device with a timer. The devices were small and crude. Police said they were still investigating the attacks.

"It is a message that Shell pumps will always be on the hit list because of being a foreign company," Shah said.

The managing director of Shell's Pakistani subsidiary, Farooq Rehmatullah, said none of the four injured staff required hospital treatment.

Militant Islamic groups angered by Pakistan's support for the U.S.-led war in Afghanistan (search) have launched a series of anti-Western attacks over the past 18 months.

Western interests have been targeted several times in Karachi, including an explosion last year outside the U.S. Consulate that killed 17 people and a suicide bombing that killed 11 French engineers outside their hotel. Wall Street Journal reporter Daniel Pearl was killed after being kidnapped in Karachi.