(Recasts, adds context on guerilla kidnappings)
By Brian Ellsworth
BOGOTA, March 6 (Reuters) - Colombia's ELN guerrillas have freed 11 oil workers who were kidnapped in late February, the International Red Cross said on Tuesday, the latest in a flurry of security incidents that could limit growth of the country's oil industry.
Although a decade-long U.S.-backed crackdown on Colombia's Marxist guerrillas paved the way for record exploration and production, security problems like hostage taking and pipeline bombings remain a top concern for oil and gas companies.
The release "was carried out at the direct request of the armed group and upon request of the families," said Michael Kramer of Colombia's delegation for the International Red Cross in a statement.
The contractors were snatched in the Arauca province as they traveled by bus to the Bicentennial pipeline which carries crude 600 miles (960 km) from Colombia's central plains to its ports on the Caribbean coast.
The Red Cross did not offer further details and the nationalities of the workers was not disclosed.
Dozens of oil sector workers have been kidnapped since the start of 2011, most of whom were taken hostage by the FARC and ELN guerrillas and later freed.
In June, three Chinese oil contractors and their translator were kidnapped by the Revolutionary Armed Forces of Colombia, or FARC, Colombia's largest guerrilla group.
The FARC, famous for high-profile kidnappings, last week vowed to stop taking hostages for ransom, though President Juan Manuel Santos said this was an insufficient step toward lasting peace.
Attacks on infrastructure were a key factor in Colombia's missing its 2011 oil production 1 million barrels per day (bpd) by about 20,000 bpd. Production in January stood at 941,000 bpd.
Extortion by armed groups, which has historically been a cash cow for both guerrillas and right-wing paramilitary groups, has become a more important concern than kidnapping.
Improved security helped attract almost $15 billion in foreign direct investment last year, most of which flowed into the oil and mining sectors. (Reporting by Luis Jaime Acosta, Writing by Brian Ellsworth; Editing by Cynthia Osterman)