Sri Lanka's navy and volunteers rescued 120 pilot whales stranded in the country's biggest mass beaching, but at least two injured animals were found dead, officials said.

Sailors from the navy and the coastguard along with local volunteers pushed back at least 120 whales by dawn Tuesday after a gruelling overnight rescue, navy spokesman Indika de Silva said.

The school of short-finned pilot whales washed ashore at Panadura, 25 kilometres (15 miles) south of Colombo, since Monday afternoon in the biggest-ever mass stranding of whales on the island.

"We used our small inshore patrol craft to pull the whales one by one back into deeper waters," de Silva told AFP. "Sadly, two whales have died of the injuries sustained when they beached."

Local authorities were braced for mass deaths as seen in Tasmania in September when about 470 pilot whales were stranded and only about 110 of them could be saved after days of rescue efforts.

Sri Lanka's Marine Environment Protection Authority (MEPA) confirmed that Panadura saw the largest single pod of whales stranded in the South Asian country.

"It is very unusual for such a large number to reach our shores," MEPA chief Dharshani Lahandapura told AFP, adding that the cause of the stranding was not known.

"We think this is similar to the mass stranding in Tasmania in September."

Pilot whales -- which can grow up to six metres (20 feet) long and weigh a tonne -- are highly social.

The causes of mass strandings remain unknown despite scientists studying the phenomenon for decades.