Mine operator PT Freeport Indonesia said 38 miners were believed to have been buried in the tunnel collapse on Wednesday, with 10 rescued and 14 still unaccounted for.
Freeport-McMoRan Copper & Gold president and chief executive officer Richard Adkerson witnessed the ongoing rescue effort over the weekend.
"I have visited the accident site and met with our injured employees, and the loved ones of those lost in this tragic event," he said.
"I am deeply saddened and disturbed by this event. Our focus continues to be continuing efforts to gain access to the victims still buried at the accident, carried out as quickly as can be done safely. The entire Freeport family around the world joins Freeport Indonesia in grieving for our lost brothers."
PTFI's emergency response team comprises 60 underground experts, who have managed to clear two passages to allow the use of heavy machinery to expedite the search and rescue effort.
"We have world-class experts and equipment being deployed to try to complete this as quickly as possible," PTFI mine general manager and emergency response team head Nurhadi Sabirin said.
He said the rock fall accident came from above the facility. Rock continued to fall from above, slowing the progress of rescuers to gain access to the victims.
The team are using the Lifepak 3 device to detect vibrations.
"This device has detected vibrations that could be consistent with a human heartbeat, but this is not conclusive and could be caused by a number of other vibrations," he said on Saturday.
However, he added that there had been no signs of life for 72 hours.
"We continue to carry out these efforts non-stop, 24 hours a day as quickly as can be done safely to do everything possible to save lives, but as more time passes the possibility of there being any survivors becomes less likely," Nurhadi said.
Progress has been slow due to the need to stabilise the ground and roof for every metre of debris cleared.
Ten workers escaped Tuesday's collapse with injuries but no one has been found alive since Wednesday.
The employees were attending an annual two-day "refresher" course in a 5m by 10m classroom built 15 years ago, well away from the main underground production area, when the tunnel collapsed.
Up to 80% of the classroom was filled with debris.
While Big Gossan is not near the mining area, PTFI immediately suspended mining at Grasberg out of respect for the dead, injured and missing workers.
The company said it had suspended union and labour negotiations for the same reason.
PTFI earlier said there was not expected to be a material impact on mining, smelting or other development activities.
The massive mine, which has been prone to violent unrest and strike action, employs more than 23,000 workers and contractors.
Phoenix-based Freeport recently reported March quarter production of 219,000 tonnes of copper and 212,000 ounces of gold from Grasberg.