(CNN) -- At least 20 people were killed and six injured Wednesday in a mine explosion in northeastern Colombia near the border with Venezuela, the government-run Colombian Institute of Geology and Mining, known by the acronym Ingeominas, reported.
"At this time, personnel from Ingeominas are at the scene of the accident dealing with the emergency with a group of institute rescue workers aided by the 5th Brigade of the national army, who brought in a helicopter for possible rescue efforts and to transport the victims," the institute said in a written statement.
Earlier, a spokeswoman for the national civil defense agency said that more miners could still be in the mine, although an exact number was unknown. Hellen Garcia said that authorities believe as many as 30 could have been affected by the explosion.
Garcia said five injured people were taken to two hospitals, but one of them died. Four bodies were taken out of the mine, she said.
The remaining miners are 1,200 meters, or almost 4,000 feet, below ground, and the rescue operation by civil defense, Red Cross and government crews was expected to last through Thursday, Garcia said.
Yamile Rangel Calderon, mayor of the municipality of Sardinata, said the mine is known as La Preciosa, or The Precious One.
"This is one of the oldest mines in the region and one that has some of the best safety and quality controls," she said. "They have been exporting coal for many years."
Calderon identified five of the miners who died as Manuel Aparicio, Gustavo Mendez, Gustavo Garcia, Jorge Luis Valdivieso and Nelson Velosa Fuentes.
The blast happened around 7 a.m. in the district of San Roque, Alvaro Silva, a government official in Sardinata, told the government radio service.
An accumulation of methane gas apparently caused the explosion, the governor of the Colombian state of North Santander told the radio service.
A 2007 explosion in the same coal mine killed 31 people, Garcia said.
CNN's Rafael Romo and CNN en Español's Claudia Dominguez contributed to this report.