"Ground and intelligence sources have confirmed that the Tigers are preparing to launch a major offensive south of Maavilaru, Kadjuwatte and Mankerni areas," the defence ministry said in a statement.
The military said the Liberation Tigers of Tamil Eelam (LTTE) had launched artillery salvos aimed at trapping civilians and had prevented food supplies from reaching several rebel-held villages.
"Continuous Tiger artillery shelling and mortar fire from Kadiraweli and Vakarai towards the Kadjuwatta and Manrkerni army camps have prevented food supplies reaching Vakarai," the ministry said.
The LTTE denied the charge and said it was the security forces who started shelling rebel-held areas and carried out an air attack on Monday afternoon.
"The military is shelling civilian villages and there were a couple of air raids this afternoon," Tiger military spokesman Rasiah Ilanthiriyan said by telephone when contacted in the rebel-held town of Kilinochchi.
He said the troops had also fired mortar bombs in a bid to prevent civilians from crossing the de facto front lines to buy provisions.
"If the military says that we started the firing, all they have to do is allow cease-fire monitors into the area so that they can make a determination," Ilanthiriyan said.
He said civilians in rebel-held areas were dangerously running low on food supplies and blamed the military for imposing an unofficial blockade.
Sri Lanka's key foreign aid donors -- the United States, European Union, Japan and Norway -- have asked both the government and the Tigers to refrain from military action even as peace talks between the combatants collapsed in Geneva last month.
Despite a truce in place since February 2002, nearly 3,300 people have died in fighting in the past year. Still, neither side has renounced the truce and both say they have only acted in self defence.
The government has said the Tigers aim to step up attacks ahead of the birthday of their supremo Velupillai Prabhakaran on November 26.
His birthday coincides with the rebels' "Heroes' Week", which commemorates the 18,500-plus guerrillas killed in the three-decade-old conflict for a separate homeland for the Tamil minority in the Sinhalese-majority nation.
The first Tiger activist to be killed by security forces was shot and wounded on November 21, 1982, and died a few days later on November 27, the date that Prabhakaran delivers his annual policy statement.
More than 60,000 people have been killed in Sri Lanka's drawn-out Tamil separatist conflict since 1972.

Copyright AFP (Agence France-Presse), 2006