The clashes were in the district of Batticaloa on Thursday, the ministry said.
"Due to the successful retaliation, troops confirmed that 18 Tigers were killed," the ministry statement said. "Troops were also able to recover a stock of arms, ammunition and war-like materials."
It did not say if government troops suffered any casualties.
The latest clashes came as a British envoy left Sri Lanka after a three-day visit aimed at reviving the island's stalled peace bid. British member of parliament Paul Murphy is expected brief Prime Minister Tony Blair on the outcome of his visit, diplomats said.
The two sides have been trading long-range fire in northern and eastern regions in recent weeks, further damaging a truce agreed to in February 2002.
An upsurge in fighting in the past year in the island's separatist conflict has claimed more than 3,300 lives.

Copyright AFP (Agence France-Presse), 2006