COLOMBO, SRI LANKA, OCT. 18 -- Indian troops were reported to have fought their way into the northern Sri Lankan city of Jaffna for the first time today in their offensive against Tamil guerrillas.

At the same time, Indian officials said that fresh troops were being ferried in by air from India to reinforce the peace-keeping forces here and that the end of the offensive against the Sri Lankan Tamil separatists was still not in sight.

"It is not and cannot be a swift, lightning type of operation," said Lakshmi Puri, a spokeswoman for the Indian government here. When the operation was launched Oct. 10, Indian officials said privately that it was not expected to last more than a week.

"The Indian peace-keeping force is fighting as if it had one hand tied behind its back," the Puri said. She said the advancing Indian infantry has not used heavy armor, extensive artillery or air cover in order to minimize casualties in the coastal city, which has an estimated peacetime population of 150,000.

It remained unclear tonight how far the Indian force had advanced into Jaffna. The force's total strength in the country is estimated to number about 20,000.

Puri claimed that "steady progress" was being made in India's four-pronged assault on the city, which has been a bastion of the Liberation Tamil Tigers of Eelam, the largest and most militant Tamil separatist group in this island nation off the southern tip of India. But she reported that the Indian troops are still 1 1/2 to 2 miles from the center of the city.

An Indian government spokesman in New Delhi tonight claimed that the Indian forces had broken through the Tigers' inner defense perimeter from the east and west and that Indian troops were "now poised to link up" with Jaffna Fort, the city's military garrison, where another Indian force was seeking to push out.

The spokesman in New Delhi claimed that Indian units had captured the main bus stand in the city center. But, he said, determined Tamil guerrillas "keep reappearing in positions they earlier abandoned," apparently behind advancing Indian forces. The main Indian casualties have been caused by extensive mines, booby traps in roads and sniper fire.

Indian officials said the reinforcements totaled between 1,500 and 2,000 additional troops. But well-informed diplomatic sources said that up to six battalions, totaling 3,600 troops, were being airlifted.

While Indian government officials continue to speak of the Tamils using civilians as "shields" against the advancing Indians, refugees who fled the city and arrived here by bus denied that they were being forced to remain in Jaffna. As of yesterday, rebel announcements -- by megaphone and on streetside blackboards -- had advised all civilians to flee because the Tigers could no longer defend them against the Indian troops, the refugees said.

The refugees, none of whom wanted to have their names used, claimed that the Tamil separatists had warned that because the Indians would not agree to a cease-fire, the rebels were prepared to continue fighting.

Indian officials today said more than 102 Indian soldiers and officers had been killed and more than 332 wounded so far in the offensive. They claimed up to 527 guerrillas had been killed.

Neither the Indian claims nor the refugee reports could be verified firsthand because Indian forces have prevented all independent observers, including foreign reporters, from traveling to Jaffna.

The Indian troops came to Sri Lanka as a result of a July agreement between Indian Prime Minister Rajiv Gandhi and Sri Lankan President Junius Jayewardene.

The agreement is aimed at ending a four-year civil war between the island's Buddhist Sinhalese majority and the mostly Hindu Tamils. Tamils make up about 13 percent of Sri Lanka's population of 16 million.

The Indian peace-keeping force was supposed to help guarantee the cease-fire and disarm the Tamil guerrillas. While smaller, less militant separatist groups agreed to support the accord, the Tigers balked and resumed attacks on Sinhalese civilians, forcing the Indians to turn against the Tigers in a major offensive that began a week ago Saturday.

The Tigers have appealed over the past week for an immediate cease-fire, but the Indian government has refused to discuss any cessation of hostilities until the rebels agree to surrender unconditionally, give up all their arms and agree to abide by the accord's terms.