BOGOTA – The largest union at Colombia's top thermal coal mine Cerrejon has voted to strike following failed salary and benefit negotiations with the company, the union said on Monday.

Cerrejon accounts for 37% of Colombia's coal output, the world's fifth-largest exporter of the mineral, and a strike is likely to affect exports. The stoppage could begin within three days, the union said.

The two sides returned to talks on Friday, but no deal was reached and Monday was the end of a ten-day voting period during which union members could decide whether to stop work or take the matter to a tribunal. Some 3,399 union members voted for a strike and 19 for a tribunal, while 17 votes were void or blank.

The company, equally owned by BHP Billiton, Anglo American and Glencore, has offered a 2018 salary increase of 5.9%. The union originally demanded 12% and increased health and education benefits.

"The majority of unionised workers at Cerrejon voted in favour of the strike. If the company does not improve its offer we won't have any alternative but to strike because there isn't progress in the talks," Aldo Amaya, president of the Sintracarbon union, told Reuters.

The union says it represents some 4 000 of the company's 5 000 workers.

Cerrejon said its negotiators would re-double their efforts to reach a "realistic and prudent" deal with the union before the possible start of a strike this week.

"The company aspires to reach a transparent accord at the negotiating table, so we can proceed to sign a new collective contract before the deadline," Cerrejon said in a statement.

Production and exports at Cerrejon fell for a third straight year in 2017, when it mined 31.7-million tonnes of coal, as heavy rainfall affected operations, the owners said this month.

Coal is Colombia's second most important export.

The last strike at Cerrejon, which produces coal at its open pit mine in northern La Guajira province, was in February 2013 and lasted 32 days.