State-run media said last week that Tamil Tiger rebels were preparing for attacks on the capital's harbour, oil storage complex and electricity supply in response to government attacks in the north and east.
Both sides suffered heavy losses earlier this month when the army advanced into rebel-held territory but was beaten back by a major counter-attack.
"The territory captured in the northern theatre after months of operations was lost in a single day when the Tigers hit back," defence analyst Namal Perera said.
"The signs are that we are headed for a prolonged stalemate."
The fighting has raised the prospect of widening conflict in the island nation of 19.5 million that could severely damage the economy and isolate the government, former Sri Lankan diplomat Nanda Godage said.
"We could be heading in the direction of sanctions," Godage said.
The war is already affecting areas like tourism and aid flows as the economy slowed sharply in the first quarter, growing 6.1 percent