However, there was no loss of life, he said.
"A small portion of the tunnel alignment towards the north portal was damaged Thursday. Since then, excavation work has been stopped," an official of the Border Roads Organisation (BRO) told IANS, adding that work would continue in due course.
The north portal lies towards Lahaul and Spiti, while the south one is towards Dhundi near Manali.
Official sources said around 30 metres of the tunnel rooftop broke down. At the time of the incident, more than 12 employees were at work in the tunnel. "As a precautionary measure, digging work has been temporarily stopped," said an official who did not wish to be identified.
The BRO, a wing of the central government that constructs and maintains crucial road links along the international borders, has been tasked with the construction of the 8.8-km Rohtang tunnel that lies beneath the 3,978-metre Rohtang Pass.
It is an engineering marvel and a strategic project, and will ensure all-weather connectivity to Keylong in Lahaul and Spiti district.
The work is being undertaken by the BRO in collaboration with Strabag-Afcons, a joint venture between India's Afcons Infrastructure and Strabag SE of Austria.
Over 350 BRO personnel and 24 experts from Germany and Austria have been working round the clock on both sides of the tunnel to meet the February 2015 deadline.
Of the 8.8-km horseshoe-shaped tunnel, over four km has been excavated since work began in June 2010.
The Rohtang tunnel is located at altitudes ranging between 3,053 m and 3,080 m. About 70 percent of the Rohtang Pass top remains under snow, even during peak summer.
Once ready, the tunnel will be a boon for the cold deserts of Lahaul Valley, where over 20,000 people remain cut off from the rest of the country in winters owing to the closure of the Rohtang Pass.
The tunnel is projected to cost Rs.1,495-crore ($290 million).