The move indicated the troops were preparing the network for the future, the officials said, despite both countries being engrossed in discussions to dissipate a deadlock in the region. The connection would allow forward troops to contact bases in the rear.
The cables were spotted to the south of Pangong Tso lake in the Himalayan region of Ladakh, a senior government official said.
Troops from both sides have been stuck in a standoff along a 70 km-long front to the south of the lake.
Both sides are equipped with tanks and aircraft.
On Monday, a third Indian official said there had been no critical retractions or reinforcements from either nation since the foreign ministers of both countries met last week.
He said: “It is as tense as earlier.”
On the lake’s southern bank troops from both sides are only separated by a few hundred meters in some areas.
The first official said: “Our biggest worry is that they have laid optical fibre cables for high-speed communications.”
“They have been laying optical fibre cables on the southern bank at breakneck speed.”
The second official added that around a month ago similar connections were spotted by Indian intelligence bureaus to the north of the Pangong Tso lake.
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He said: “As the site of a bloody tactical battle in 1962 which India lost, before losing that year’s short conflict altogether, Pangong Tso carries considerable symbolic value.”
The tension between both nuclear-armed powers alarmed western countries after earlier this month, China accused Indian troops of illegally trespassing on Chinese territory in the Himalayas.
About the purported breach, Chinese Foreign Ministry spokeswoman Hua Chunying said: “The Indian side has severely undermined China’s territorial sovereignty, breached bilateral agreements and important consensus, and damaged peace and tranquillity in the border areas, which runs counter to the recent efforts made by both sides for de-escalation of tensions on the ground.”
Writing last month for the Carnegie Endowment for International Peace, analysts Toby Dalton and Tong Zhao defined the confrontation as “the gravest military confrontation the two nuclear powers have faced in fifty years.