Sri Lankan civilians wait for registration at a transit camp in Omantaion Wednesday. (Sanath Priyantha/Associated Press)

Two senior Tamil Tiger rebels have reportedly surrendered to the Sri Lankan military as more than 80,000 people continued to flee the war zone, according to government officials.

Tamil Tigers media spokesman Velayutham Dayanithi, also known as Daya Master, and an interpreter for the insurgents' political wing have turned themselves over to the army, military spokesman Brig. Udaya Nanayakkara said Wednesday.

The interpreter is only known as George.

Analysts told the BBC that if the report is true, the surrenders would be a major setback for the embattled rebel leadership.

The army issued at statement on its website indicating that the two men surrendered in Puthumathalan on Wednesday.

Both had played prominent roles in talking to the media and visiting foreign diplomats during a now-defunct peace process mediated by Norway.

Dayanithi is most senior rebel official to surrender so far, Nanayakkara said.

Clinton: Sri Lanka caused 'untold suffering'

Sri Lankan soldiers have pushed deep into the sole remaining Tamil rebel enclave and killed 43 guerrillas while nearly 82,000 civilians have fled the northern war zone within the last 72 hours, the military said Wednesday.

Meanwhile on Wednesday, U.S. Secretary of State Hillary Clinton said Sri Lanka has caused "untold suffering" for thousands of civilians trapped in the war zone as government forces fight the rebels.

"I think that the Sri Lankan government knows that the entire world is very disappointed that in its efforts to end what it sees as 25 years of conflict, it is causing such untold suffering," Clinton told a congressional committee in Washington.

Troops in recent months have ousted the rebels from all their former strongholds and hemmed them into what the government previously deemed a "no-fire" zone to protect civilians. But troops entered the 20-square-kilometre zone this week to fight remaining rebels and free civilians trapped there.

The military said it broke through a key rebel bunker in the coastal strip on Monday and that tens of thousands of civilians have been fleeing the area since then. By Wednesday, 81,423 civilians had escaped, the military said.

Troops advanced deep into the zone and captured part of it during fighting with insurgents on Monday and Tuesday, killing at least 43 rebels, said Nanayakkara.

Tamil rebels are cornered into a stretch of about eight kilometres, he said. Pictures released by the military on Wednesday showed the sandy beaches north of the tiny combat zone were filled with people carrying their belongings on their backs or in bundles on their heads.

Army commandos attacked a car carrying rebels and killed four Tuesday, while another four insurgents were killed by commandoes in a separate attack, Nanayakkara said. Other 35 rebels were killed in separate attacks on Monday and Tuesday.

He said government forces also suffered casualties, but did not provide details.

"We suffered casualties because we are not using heavy and long-range weapons. We only use small weapons," Nanayakkara said, accusing rebels of firing artillery shells.

It's not possible to obtain independent accounts of the situation because the war zone is off-limits to journalists.

Grave concerns about trapped civilians

The United Nations, many countries and rights groups have expressed grave concerns for remaining trapped civilians, fearing the government may launch an all-out assault soon, after giving the rebels a 24-hour ultimatum to surrender.

A government deadline of noon local time Tuesday for the Tamil Tigers to surrender passed without word from the rebel group.

The UN and others have called for a negotiated truce to allow civilians to leave the dwindling, rebel-held enclave. But the government has refused to heed such calls, saying it is on the verge of crushing the rebels and putting an end to the quarter-century-long conflict.

Tens of thousands of civilians are still confined to a rapidly shrinking territory declared a "no-fire zone" by government forces, according to the International Committee of the Red Cross.

The Red Cross has called for "exceptional precautionary measures" from both sides to minimize further mass civilian casualties. The organization has described the situation in the area as "nothing short of catastrophic."

The UN estimated more than 4,500 civilians have been killed in the past three months.

The Tamil rebels allege 1,000 civilians were killed and more than 2,000 were injured on Monday when government troops raided their last sliver of territory

The Sri Lankan military has denied the allegation. The military and human rights groups have accused the rebel group of targeting civilians leaving the conflict zone in a desperate attempt to use them as human shields against advancing government troops.

Number of refugees underestimated

The number of fleeing civilians made it clear that the government had vastly underestimated how many people were caught in the fighting. While aid groups had estimated that about 100,000 civilians were trapped ahead of this week's exodus, the government had said the figure was only about 40,000.

About 7,500 civilians have reached refugee centres away from the front in Jaffna and Vavuniya, according to the UN.

The Tigers have been fighting since 1983 for an independent state for the Tamil minority, which they say suffered decades of marginalization at the hands of governments dominated by the Sinhalese majority. An estimated 70,000 people have been killed in the violence.

Government forces say they are close to crushing the separatists after a blistering offensive in recent months in the country's war-ravaged north.

The rebel group is officially banned in Canada since the Conservative government listed the Tigers — known formally as the Liberation Tigers of Tamil Eelam (LTTE) — as a terrorist organization in 2006 for its reported use of suicide bombers and child soldiers during the civil war.

With files from The Associated Press