Human Rights Watch said on Friday that up to 2,000 non-combatants had been killed and accused both sides of war crimes, calling on them to immediately stop "the ongoing slaughter of civilians".
For its part, the UN says it is deeply concerned for thousands of people trapped in the island's northeast.
John Holmes, the UN humanitarian chief, expressed concern for the trapped civilians at the end of a three-day visit to Sri Lanka.
"I fear the reality is that significant numbers of people are still killed and injured every day in that pocket," he said on Saturday.
Holmes, who met high-level government officials and visited displacement camps in Vavuniya in the northeast, said not enough food and other aid was reaching the civilians.
He raised concerns about the heavy military presence at the camps for the more than 30,000 civilians who have fled the war zone.
Holmes promised that the UN would send $10 million in emergency aid including food, medicine, shelter and other necessities for the displaced civilians, and urged both sides to ensure an orderly end to the conflict.
During his trip, Holmes also condemned the recent kamikaze-style attack on Colombo and urged both sides in the civil war to avoid a "final bloodbath".
The LTTE launched an air assault on the country's capital on Friday night, killing four people, including the pilots, and wounding 51 others.
The wounded included tax department staff and bystanders hit by falling debris after one of the LTTE aircraft hit a building housing the inland revenue department.
The second aircraft was shot down by ground-based gunners near Colombo's international airport.
The LTTE is believed to have had five Czech-built Zlin-143 aircraft smuggled into the country in pieces and re-assembled.
Military officials said another 17 LTTE fighters were killed in ground battles in the island's north on Friday.
The defence ministry also said LTTE fighters killed 10 civilians, including two children, at a village in the east of the island on Saturday.
The authorities released a navy video of the LTTE air raid at a news conference on Sunday, with a military spokesman asserting that the government succeeded in preventing the two aircraft from bombing Colombo.
Our correspondent explained that while the LTTE claimed its suicide mission was a success, the government was at pains to stress that casualties would have been much worse if it had not been able to shoot the aircraft down.
"It was a very embarrassing attack on the nation's capital, that two planes were able to cross most of the island without being intercepted along the way," Hawkins said.
He said the military confirmed it was aware that the LTTE aircraft had taken off but was unable to locate them before they reached Colombo "because they flew so low and so slowly".
Authorities have since stepped up air defences, with police and military officials saying they expect more LTTE attacks as the group steadily loses territory to advancing government forces.
The government has repeatedly said it is on the verge of defeating the LTTE.
It says the LTTE has now lost over 98 per cent of the territory it once controlled and is confined to an area of less than 100sq km along a coastal jungle stretch in the northeast.