Two people were killed and four badly injured Friday in an explosion on an oil rig in the Gulf of Mexico, the U.S. Coast Guard said.

Authorities have been unable to account for two more of the 28 workers on the platform at the time of the blast.

The rig lies in fairly shallow water around 40 kilometers (25 miles) south of Grande Isle, Louisiana.

"Preliminary report we're getting is that there were some workers doing maintenance and construction," Coast Guard Capt. Peter Gautier told WWL-TV in New Orleans. "It sounds like a pipe was cut into that might have had some oil in it that caught fire."

The four injured workers were taken by helicopter to a hospital in Louisiana.

"We're not facing, from what we can see, a major oil spill response here. We're seeing a small sheen but it's not a major environmental issue," Gautier said.

The rig is operated by Houston-based Black Elk Energy.

The explosion came a day after BP Plc agreed to pay $4.5 billion to settle criminal charges arising from the April 2010 blast on its Deepwater Horizon platform in the Gulf, an accident that caused 11 deaths and the worst offshore oil spill in U.S. history. EFE