Government troops have made dramatic progress on the northern fronts in the past months, seizing a series of rebel bases and chunks of land.
But the rebels have offered stiff resistance as government troops approach the edge of Kilinochchi, 330km north of Colombo, the Sri Lankan capital.
The government earlier this month said it was poised to capture Kilinochchi.
That drive, however, could take longer than anticipated with the rebels digging in, analysts and military sources say.
"Troops operating in the south of Kilinochchi are making progress, though they have not shown a rapid progress within the past few days," Ranil Wijayapala, a defence analyst writing in the state-run Daily News, said.
"With the rapid advance last month, the military is also forced to tie down a lot of soldiers to protect areas newly captured from the Tigers," a military officer, who declined to be named, said.
"The Tigers won't be able to hold Kilinochchi for too long, but they can delay the fall."
The government has vowed to crush the rebels and end their 25-year campaign for a separate homeland for the island's ethnic minority Tamils, who have suffered marginalisation by successive governments controlled by majority ethnic Sinhalese.
More than 70,000 people have been killed in the conflict so far.