Union leaders told Reuters Wednesday morning that votes would be counted starting at 7pm local time and that the offer from the producer was close on pay but far off on proposed job cuts.

Sintramienergetica is demanding a 10.7% pay increase and a one-time bonus of 7 million pesos ($US3700), among other items.

Drummond’s most recent offer was a 4.5% raise on top of a one-time bonus.

“The majority will vote for a strike because they're seeking better proposals from the company," Sintramienergetica vice president Edgar Munoz said.

Should Drummond workers walk off the job, it would be the second major stoppage for the Colombian coal industry this year. In February hundreds held a month-long strike for better pay and conditions from Cerrejon.

US-based Drummond produces more than a quarter of the nation’s coal annually, so the two strikes together could potentially take a bite from Colombia’s economic growth.

A strike of any length by the group could also raise the price of European coal at a time when buyers are examining winter pricing.

Adding to the issue, Reuters said the Andean nation’s artisanal and small-scale miners began a strike of their own Wednesday, demanding operating licenses that would legitimize their activities.

“We have been forced to beg in our own territory for our own riches,” small miners' union Conalminercol spokesman Ruben Dario Gomez said.

As for the potential Drummond strike, Munoz said it would likely begin after Friday and before the end of next week – the window for action outlined under federal law.

Still, the action could be called off if the parties make a deal.

“Even after [Wednesday], there is still a possibility of reaching an agreement,” Munoz said.

"We understand the importance of this for the country and the government's royalties.

“Striking means a big economic loss for the workers and for the company. That is why we are trying to find a solution without a strike.”

Drummond has not commented on the stage of talks or possible walkoff.

The company employs about 10,000 workers across its main Colombia operations. About half are direct hires represented by Sintramienergetica. The balance, represented by another entity, will follow the vote as well.