Sri Lankan authorities temporarily closed Colombo international airport and cut power to the city late on Thursday after suspicious airplanes were seen flying south along the coast, the military said.
Witnesses said they saw parachute flares fired into the sky and heard what sounded like anti-aircraft guns, and flag carrier SriLankan Airlines diverted three inbound international flights.
The air raid scare came two days after the Tamil Tiger rebels' newly unveiled air wing staged its second attack ever, dropping bombs on a military position in the north killing six people. The rebels' first air strike was on the air force base next to Colombo airport, and it took the military by surprise.
"Some civilians in Puttalam district had seen three aircraft flying from north to south hugging the sea. With suspicion, contingency plans have been activated because of the imminent risk of threat, closing off the entrance of the airport and switching power off," a senior military source said.
Puttalam is about 130 km north of Colombo on the western coast of the Indian Ocean island.
Analysts believe the Tamil Tigers' air force consists of just two to five light propeller planes assembled from pieces smuggled in over time.
"Only the air defence system is activated, that is all. There was information saying there is suspected aircraft movement, so the air defence system is on all over the place and we are also doing a search," air force spokesman Ajantha de Silva said.
In the central town of Anuradhapura, a helicopter gunship scrambling to intercept the suspect planes was forced to make an emergency landing, but the two pilots were safe, he said.
The Liberation Tigers of Tamil Eelam, as the rebels are officially known, want to create an independent state in the north and east of the island for ethnic minority Tamils.
Since 1983, the war in Sri Lanka has claimed some 68,000 lives, including more than 4,000 since late 2005. The intensified violence of the past 16 months has left a 2002 ceasefire in tatters.
Chandana de Silva, a spokesman at SriLankan Airlines, said the airport was closed from about 10:40 pm till 11:40 pm. Three flights from Bangkok, Delhi and Cochin had been diverted to Trivandrum.
"Since the airport is open now, we might have to refuel these aircraft and then come back," he said.
After the rebels' first air attack, which killed three airmen and wounded 16, Cathay Pacific suspended flights to and from Sri Lanka for almost a month. The airport is 37 km north of Colombo.
Earlier on Thursday, air force jets attacked what the military said was a gathering of Tamil Tiger leaders in the rebel-held north, but the rebels denied leaders were present.
In 2001, Tigers commandos attacked Colombo international airport, destroying half of SriLankan Airlines' fleet of planes and several military aircraft.