Two contract workers on a $4 billion oil pipeline being built in northeastern Colombia near Venezuela have been kidnapped allegedly by leftist guerrillas, RCN Radio said Wednesday, citing a police official.

"We don't know why they were taken, but we're undertaking operations to locate them," Col. Wilson Bravo, police commander in the rural state of Arauca, told the radio station.

Col. Bravo said it appears Tuesday's kidnapping was the work of Colombia's second-largest rebel group, the National Liberation Army, or ELN, but said the country's main rebel group, the Revolutionary Armed Forces of Colombia, or FARC, may have also been involved.

The Bicentennial Pipeline, or OBC, is 55%-owned by state oil company Ecopetrol SA (EC, ECOPETROL.BO), while Toronto-based Pacific Rubiales Energy Corp. (PEGFF, PRE.T) also has a stake. When the first phase is complete, the pipeline will carry 120,000 barrels a day of crude oil 600 miles to the Covenas port on the Atlantic, from Araguaney, the offloading station for the eastern Llanos basin.

The workers kidnapped, Elida Parra and Gina Paola Uribe, were contracted by Ecopetrol to do social work in the community where the pipeline is being built, the radio station said. They were kidnapped from their homes in the town of Saravena by men on motorcycles, it said.

The pipeline was initially expected to be finished already, but Ecopetrol pushed the finish date back several times, and most recently said pumping won't begin until early 2013.

In March, 11 workers involved in building the pipeline were also kidnapped by the ELN, but they were freed a week later.

Due to an improved security situation, Colombia saw a boom in its oil sector between 2007 and 2011, with production reaching a record 962,000 barrels a day of crude oil in November, making it Latin America's fourth-largest producer.

But production has declined since then, to 934,000 barrels a day in June, as leftist rebels have begun a resurgence. Oil infrastructure has again become one of their favorite targets.

Write to Dan Molinski at dan.molinski@dowjones.com