"The company did not improve its offer. At 6 p.m.today we began an unlimited strike," Edgar Munoz, vice president of National Union of Mining and Energy, told reporters after the union and U.S.-based Drummond failed to reach an agreement.

　　Union treasurer Dario Mosquera said that Drummond workers are demanding increased pay and oppose hundreds of planned layoffs in the mining sector.

　　"We are concerned that company's reform will lay off 600 workers," Mosquera said.

　　Colombia is the world's fourth-largest coal exporter. A three-week strike earlier this year caused the country's coal production to fall 21.4 percent in the first quarter.

　　Drummond produced 26 million tons of coal, about one third of Colombia's total output, in 2012.