COLOMBO, SRI LANKA, OCT. 15 -- A mine exploded today under an Indian Army troop truck in eastern Sri Lanka, killing 20 soldiers in the largest single casualty toll suffered by the peace-keeping force since it arrived here July 30, while an Indian offensive against the rebel-held northern city of Jaffna appeared to stall.

The bogged-down Jaffna offensive and the mine blast were setbacks to Indian hopes of striking a quick, decisive blow against the guerrillas of the Liberation Tigers of Tamil Elam. The Indians originally came to Sri Lanka to assure the security of the rebels as they disarmed.

While Indian officials acknowledged tonight that 79 Indian troops have been killed in recent days and that at least 17 are missing, a senior Sri Lankan official with close ties to the military said the toll was higher -- at least 108 Indians killed and "two truckloads of troops missing."

He said the Indians were playing down their casualties because of embarrassment at how high their losses had been.

Laxmi Puri, a spokeswoman here for the Indian High Commission, or embassy, said Indian forces near the rebel stronghold of Jaffna for the most part were "consolidating" their positions. She said there was "stiff resistance" along most of the Indian line of attack and she believed there were plans to bring in more troops from India to reinforce the drive.

Puri said she had no information on civilian casualties in Jaffna, where India claims the rebels are holding the population hostage for use as "shields" against the Indian offensive.

The city is described as without electricity, its hospital overflowing and said to be damaged by shell fire, and with food scarce because all the shops have been shuttered since the fighting began.

Puri placed Tamil guerrilla casualties at 380 and said monitored rebel radio communications had mentioned that "a large number" of rebels had been killed and that 18 Indian soldiers had been taken prisoner.

The Indians were using some of their crack troops, including Nepalese Gurkhas and other elite forces, to attack Jaffna, a city of twisting streets and alleys.

"Jaffna Town is a very difficult nut to crack," said a western diplomat who visited the city earlier this summer. "If the guerrillas decide to fight to the death as they had vowed, I would not be surprised if Jaffna Town is almost flattened."

That was also the opinion of some senior Sri Lankan officials, who are worried about Indian anger at having lost more soldiers than at any time since the 1971 war with Pakistan that resulted in the birth of Bangladesh.

"The Indian Army is in a bloody rage because they have lost so many people," said one senior officer, who asked not to be named. "I think {Indian Prime Minister} Rajiv Gandhi himself couldn't stop them, even if he wanted to.

"I fear that by the weekend the Indians will have killed 3,000 civilians in the city, because they are using a lot of heavy weapons to bombard it," he said. "They will level the town."

Part of the Indian anger stems from two tactical defeats. One was on Tuesday when 22 commandos were wiped out after having been helicoptered into the heart of Jaffna. The other happened today, when what officials said was a remote-controlled mine was triggered as a seven-truck troop convoy was passing near the town of Batticaloa, in the heavily Tamil-populated eastern part of the country.

The two incidents have raised questions about the effectiveness of the Indian forces. It was unclear why the Indians would have dropped commandos into the heart of the Tamil Tigers' Jaffna bastion without adequate support and why, after years of experience with mined roads, they were driving down a highway with trucks full of troops.

The estimated 6,000 Indian troops attacking Jaffna are part of a force of about 15,000 soldiers and police who were airlifted here to enforce a July 29 cease-fire negotiated under Indian auspices to end a four-year Tamil guerrilla war against the island's Sinhalese majority.

Like the Indians, the Tamils of Sri Lanka's northern and eastern provinces are Hindus. The Tamils have historic and ethnic ties to the 45 million Tamils who populate large parts of southern India, especially in the state of Tamil Nadu. The Sinhalese, who are mostly Buddhists, make up 74 percent of Sri Lanka's population of 16 million.

The Tamils long have felt discriminated against by the Sinhalese. Since 1983, at least five groups of guerrillas have been fighting in Sri Lanka's predominantly Tamil provinces in the north and east for autonomy or an outright secessionist Tamil state, to be named Elam.

When the cease-fire was signed by Gandhi and Sri Lankan President Junius Jayewardene, India was viewed as having maneuvered itself into Sri Lanka as the defender of its fellow Hindus, the Tamils. Today, India is perceived by the Sinhalese as having taken over their long, bitterly criticized struggle against the secessionists.

"We have got them to do the job of our forces," said one senior Sinhalese official here, recalling how India had criticized Sri Lankan efforts to suppress the secessionists in the past.

The July 29 accord sought to end the civil war by ordering a cease-fire, bringing in the Indian forces to guarantee Tamil security while they disarmed, and setting up a new political structure in the north and eastern regions to guarantee Tamil rights.

Four of the five Tamil secessionist groups opposing the Sinhalese were quickly brought into line by Indian political pressure. But the Tigers, the biggest and most organized guerrilla movement, balked, refusing to surrender most of their weapons and last month renewing attacks on Sinhalese civilians and those Tamils who oppose them. This act angered the Indians, who had brokered the peace accord.