Sri Lanka's elite anti-terrorist commandos seized a truck packed with explosives when they overran a key Tamil rebel base in the country's east.
The military said the vehicle was destined for targets in the capital.
Separately, the UN warned that renewed fighting has left thousands of civilians cut off from food and other basic needs in eastern Sri Lanka.
The Defence Ministry said on Tuesday 52 Tamils from 15 families crossed over to government-held areas overnight in the region.
Brig Prasad Samarasinghe, the military's spokesman, told reporters that a rebel base in eastern Amparai district fell on Monday to the Special Task Force personnel, who also recovered a large quantity of arms and ammunition.
Rebels were planning to use the truck and the explosives to carry out suicide attacks in the capital of Colombo, the Defence Ministry said on its website late on Monday.
Special Task Force chief Nimal Lewke said four personnel were wounded during the operation, and that a search was on for rebel fighters who escaped.
Eastern Sri Lanka has become a hotbed of violence between the military and the Tamil Tigers, who have been fighting for over 20 years for a separate homeland for Sri Lanka's 3.1 million minority ethnic Tamils after decades of discrimination by the majority Sinhalese.
Both sides claim to be adhering to a 2002 ceasefire, but violence has escalated since late 2005, with over 3,600 people killed last year.
Lewke said guerrillas had used the Ampara base to launch attacks on government forces and as a training camp for new recruits. He said it also had a hospital.
Rebel spokesman Rasiah Ilanthirayan confirmed a battle in the area but denied that the base had fallen.
"They (the Special Task Force) have been trying to infiltrate our area and there was also a confrontation.
But I deny that the camp has been captured," he said by telephone from the rebel stronghold of Kilinochchi.
The United Nations, meanwhile, said 15,000 Tamil civilians have been trapped by fighting in rebel-held Vaharai, in the eastern Batticaloa area.
"These persons are the most vulnerable of the vulnerable. We must always recall that it is the most weak who remain behind — the elderly, the sick and the disabled," a UN statement quoted Amin Awad, Acting Resident and Humanitarian Coordinator, as saying.
The government says about 20,000 civilians have moved into government-held areas to escape violence.
Awad said no food, medicine or other humanitarian supplies had been allowed into Vaharai since November 29.