Indian rescuers have drilled two-thirds of the way through debris towards 41 workers trapped in a collapsed road tunnel, officials said Wednesday, warning that the next 24 hours could be critical.

Engineers are working to drive a steel pipe through at least 57 metres (187 feet) of the tonnes of earth, concrete and rubble that has divided the trapped men from freedom since a portion of the under-construction tunnel in the northern Himalayan state of Uttarakhand collapsed 11 days ago.

Looking into the Silkyara tunnel entrance on Wednesday, an AFP journalist could see sparks flying as workers welded metal tube sections together, with the site busy as excavators and heavy trucks brought in equipment.

"I am very happy to share... that 39 metres of drilling has been completed," said Mahmood Ahmad, a road and highways ministry official involved in the operations.

"If there is no blockage, we hope there could be happy news late tonight or tomorrow," Ahmad told reporters at the site.

"We are moving forward at a fast pace," he added.

"Stay hopeful, pray that the pace continues."

'Challenging Himalayan terrain' 

However, he warned that the remaining section yet to be drilled was critical.

Rescue efforts have been slow, complicated by falling debris as well as repeated breakdowns of crucial heavy-drilling machines.

The giant earth-boring machine last week ran into boulders, and drilling was put on hold for more than three days after a cracking sound in the roof.

Uttarakhand chief minister Pushkar Singh Dhami on Wednesday spoke of the "positive progress made in the last 24 hours", without further details.

But a government statement also noted that "timelines provided are subject to change due to technical glitches, the challenging Himalayan terrain, and unforeseen emergencies".

In case the route through the main tunnel entrance does not work, blasting and drilling have also begun from the far end of the unfinished tunnel, nearly half a kilometre (over a quarter of a mile) long. Preparations have also been made for a risky vertical shaft directly above.

On the forested hill above, rescuers have cut an entirely new track to bring heavy equipment above the men, where a machine to dig an 89-metre vertical shaft is being installed -- a complex dig above the men in an area that has already suffered a collapse.

The workers were seen alive for the first time on Tuesday, peering into the lens of an endoscopic camera sent by rescuers down a thin pipe through which air, food, water and electricity are being delivered.

'High spirits' 

Though trapped, they have plenty of space, with the area inside 8.5 metres high and stretching about two kilometres in length.

Bhaskar Khulbe, a senior government official overseeing rescue efforts, said he had spoken to the trapped men on Wednesday morning.

"All of them are in high spirits," he said.

On Wednesday, a Hindu priest sat close to the site holding prayers for the trapped men, with rescue workers pausing briefly at a shrine erected at its entrance.

The tunnel is part of Prime Minister Narendra Modi's infrastructure project aimed at cutting travel times between some of the most popular Hindu sites in the country, as well as to improve access to strategic areas bordering rival China.

But experts have warned about the impact of extensive construction in Uttarakhand, large parts of which are prone to landslides.

"The development model in this young, fragile mountain range has been disastrous and needs course correction," environmental campaigner Priyadarshini Patel wrote in the Times of India on Wednesday.

"Mega-projects are not what the Himalayas are about, culturally or geologically".