The Sintramienergetica union, which represents more than half of about 10,000 Drummond miners and port workers, walked off the job late Tuesday evening local time after talks with US-based Drummond failed.

The government is examining the impact the strike action will take on its export output.

It said, earlier this week, that tonnage would be cut by as much as one-third daily.

“I'm calling on the company and the workers to strive today to take their dialogue forward and hopefully, for the benefit of all, we reach an agreement,” Renjifo told Reuters.

Both of Drummond’s mines have halted production, and ships are not leaving the company’s nearby port facility. Together, the mines produced 26 million tonnes in 2012.

Union vice president Edgar Munoz told Reuters that, as of Wednesday evening, no contract had been made with the company, which had offered a 4.75% wage increase plus a one-time $3700 bonus.

Union workers are seeking a payrise of about 10% with multiple yearly lump sums.

Sintramienergetica already said it was prepared to suspend the walk-out if Drummond made a satisfactory offer to its members.

As the stoppage continued, Renjifo told Reuters there were two ships berthed at Drummond's port that could not depart until the strike was lifted, as the port workers were included in the action.

He also said Drummond, based in Alabama, had been expected to produce about 32 million tonnes out Colombia’s planned 94Mt goal in 2013, which would yield the Andean nation about 900 billion pesos ($475.7million) in royalties.

It made 700 billion pesos ($370 million) last year.

Next to Cerrejon, Drummond is one of Colombia’s biggest coal players.

The country is the world’s fourth-largest coal exporter.