A soldier made a brief incursion on Wednesday into the flooded coal mine in northern Mexico where 10 workers have been trapped for a week.
Equipped with an oxygen tank and dressed in a uniform and helmet, the soldier descended into one of the mine shafts in a metal basket, emerging minutes later with his clothes visibly wet.
The governor of Coahuila, Miguel Riquelme, later said on Twitter that a military diver had descended into shaft number four of the mine, but encountered "obstacles in being able to enter the galleries."
Riquelme said efforts to pump water out of the mine would continue.
Five workers managed to escape in the initial aftermath of the accident on August 3, but there has been no contact with the others.
The civil defense system said on Twitter that drone flyovers had been conducted to "map the location of worksites and obtain georeferenced information" from the rescue area.
Two underwater drones have also been deployed in the operation in Agujita, in the northern state of Coahuila, as have hundreds of soldiers and other rescuers, 25 water pumps and seven drills.
According to authorities, the flood occurred as miners were carrying out excavation work and hit an adjoining mine full of water.
The focus so far has been on pumping out water from the 60-meter (200-feet) deep, crudely constructed mine.
The water in the shafts had fallen significantly, from more than 30 meters, but needed to be reduced by several meters more before it was safe to enter, civil defense national coordinator Laura Velazquez said earlier on Wednesday.
Velazquez said that "all rescuers have the necessary equipment to be able to enter at any time."
But the news was greeted with caution by anxious relatives.
"Let's hope that now it's true. Every day they say the same thing," said Juan Orlando Mireles, whose father is among the missing.
Five days ago, soldiers cordoned off the rescue area from journalists and relatives. From behind the fence, it is difficult to observe the rescuers' actions.
In a region parched by severe drought, the amount of water pumped out of the mine over the past seven days has come as a surprise.
Mireles, a miner like his father, said it could be due to the proximity of the Sabinas River and the old Las Conchas mine, abandoned more than 30 years ago, where a huge amount of water could have accumulated.
Unlike small artisanal mines such as the one where the accident occurred, industrial mines like Las Conchas have long subterranean tunnels where water can build up, he said. They also tend to have reinforced walls, unlike the smaller mines.
According to governor Riquelme, there were no updated plans available for the flooded mine, which is operated by a private company.
The local prosecutor has announced an investigation into the accident, the likes of which are common in the state.
Coahuila, Mexico's main coal-producing region, has seen a series of fatal mining incidents over the years.
The worst was an explosion that claimed 65 lives at the Pasta de Conchos mine in 2006.
Last year, seven miners died when they were trapped in the region.