BOGOTA - A days-long road blockade by protesters has halted production at Zijin Mining's Colombia gold project, the Chinese company said on Tuesday, as authorities worked to determine whether informal miners are trapped in tunnels on its land.

The blockades began on August 12 as the protesters demanded the national mining agency's rescue team verify if miners are stuck underground, Zijin has said.

But efforts by the team were partially stymied by unidentified people who threatened the group, the government of Antioquia province said late on Tuesday.

Thousands of wildcat miners work in sometimes-deadly conditions in dozens of informal tunnels in Buritica municipality in northwestern Colombia, including many within or adjoining Zijin's concession.

The activities, controlled by the Clan del Golfo crime gang, are a safety issue for the surrounding community and affect Zijin's output, a Reuters investigation last year showed. 

"At noon the team of experts in mine rescue finished their tour and humanitarian verification of the places where it had been indicated, supposedly, bodies of disappeared miners were to be found," the statement from the provincial government said.

The group went some 600 m into the tunnels, the statement said.

A local official then proceeded a further 50 m to speak with "unidentified people who rejected the continuity of the humanitarian action with threats and said tomorrow they will hand over the bodies being looked for."

The provincial government will hold a meeting with national officials on Wednesday to discuss the situation, the statement said.

A blockade of the Pan-American highway was lifted by protesters, it added, but local roads into Buritica remain blocked.

Its production has been interrupted, Zijin said earlier on Tuesday.

The entrance to its site has been shuttered by the blockades, vehicles transporting contractors have been attacked and guard posts vandalized, the company has said.

Local media reported late last week dozens of miners could be trapped, but neither the province nor the national mining agency has confirmed reports of the missing, they have said.

A Buritica resident told Reuters the town could soon experience food shortages because of the blockade.

Zijin paid $1-billion in late 2019 to buy the mine from Canada's Continental Gold, despite security concerns related to attacks on staff.