A bullish government earlier this month said it was poised to capture the Tigers' Kilinochchi powerbase, 330 kilometres (206 miles) north of Colombo.
That drive, however, could take longer than anticipated with the separatist Tigers digging in, an army stretching to defend territory already seized, and poor weather hampering operations, the sources say.
Security forces had banked on a quick fall of Kilinochchi, the Liberation Tigers of Tamil Eelam's (LTTE's) political capital.
In early October the army said it was within two kilometres, but since then there has been little forward movement and monsoon rains have also slowed the troops' advance.
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The increased resistance in recent weeks is in sharp contrast to an earlier LTTE fallback that enabled troops to move rapidly into rebel territory.
"Troops operating in the south of Kilinochchi are m