Drummond and union Sintramienergetica failed to reach an agreement when they returned to the table on Tuesday after 14 days off the job.

Sintramienergetica represents about 10,000 Drummond workers and is one of three unions staging the strike that began on July 23.

Drummond had sought face-to-face talks with the three striking unions but Sintramienergetica wanted to negotiate alone and rejected a written offer via the labor ministry, Sintramienergetica vice president Edgar Munoz told Reuters.

"There was no agreement because there was no direct contact made with the company,” he reportedly said.

"We won't accept these sorts of negotiations.”

Munoz added that the union was willing to continue to try reaching an agreement with the company.

Workers are seeking a 9% wage increase with subsequent inflation-related raises.

The company increased its offer to raise workers' pay by 5% for the first year, from its earlier proposal of 4.75%.

It also improved its offer for a one-time bonus to about $4500 from $3700 earlier.

Drummond met representatives of the other two unions but Sintramienergetica "refused to participate", the company said in a statement, calling its offer “very generous”

"After about 60 days of talks and two weeks of strikes, no agreement has been reached.”

Drummond produced 26 million tons of coal in 2012, about a third of the Andean nation’s total output.

The miner had been expected to produce 32Mt of the country’s projected 94Mt total forecast for this year but due to the strike the company has been forced to called a force majeure on some shipments.

Colombia is the fourth-largest coal exporter worldwide.