PYEONGTAEK, April 29 (AFP) - Flags flew at half-mast, sirens wailed and families wept as South Korea bade farewell Thursday to 46 sailors killed in a warship blast, amid suspicions North Korea was to blame.

Navy chief Admiral Kim Sung-Chan vowed retaliation against whoever was responsible when he addressed a mass funeral attended by some 2,800 people including President Lee Myung-Bak.

"We'll never forgive whoever inflicted this great pain on us," Kim said. "We will track them down to the end and we will, by all means, make them pay for this."

South Korea has not openly blamed its communist neighbour for the blast, which tore the 1,200-tonne corvette apart near their disputed border on March 26.

The area was the scene of deadly naval clashes in 1999 and 2002, and of a firefight last November which set a North Korean patrol boat ablaze.

The defence minister has said a heavy torpedo was among the likeliest causes of the sinking. The North denies involvement but has a history of attacks on the South.

Former president Chun Doo-Hwan, targeted by a North Korean bombing in Myanmar in 1983, was among those attending the funeral at a naval base at Pyeongtaek 70 kilometres (44 miles) south of Seoul.

The South, which has salvaged the two ship sections from the bed of the Yellow Sea, is mounting a multinational probe by more than 100 investigators.

Based on the outcome, "we will take resolute and stern measures", said foreign ministry spokesman Kim Young-Sun on Thursday.

Government and military officials, ambassadors, foreign delegations and bereaved families attended the ceremony. President Lee laid posthumous medals in front of portraits of the dead as their names were read out.

Buddhist and Christian funeral rites were performed before weeping families laid white chrysanthemums, a traditional mourning flower, before the portraits. One grief-stricken elderly woman briefly collapsed.

A young boy wiped tears from his mother's face.

A visibly moved Lee also laid chrysanthemums and burnt incense to honour the dead, bowing before the portraits.

Some 3,000 black and white balloons -- symbolising the colours of a naval uniform -- were released into the chilly wind.

Mourners gathered across the country, some observing a moment of silence.

"I prayed that they rest in peace in a good place," said Shin Uk-Cheol, 66, at an altar outside Seoul's City Hall. "They died sacrificing themselves for the nation."

Searchers are scouring the seabed for any remnants that could confirm an attack on the Cheonan.

Seoul has not responded militarily to previous North Korean acts of aggression -- such as the 1987 bombing of a South Korean airliner with the loss of 115 lives -- for fear of sparking a wider war on the heavily armed peninsula.

Officials have said a military response has not been ruled out if the North is found responsible, but indicated the warship case would likely be taken to the United Nations Security Council.

But Baek Seung-Joo of the Korea Institute for Defence Analyses said permanent Council members China and Russia would not agree to punitive measures without hard evidence of the North's involvement -- and this would be difficult to find.

The foreign ministry said the incident would hamper efforts to restart six-party nuclear disarmament talks involving both Koreas.

"As the sinking is a grave incident, (the six-party talks) will be affected a lot by what will come of it," the spokesman said.

After the funeral, the remains of the dead were taken to the Daejeon National Cemetery further south.

The bodies of six sailors remain unrecovered. Their families burned naval uniforms, personal belongings or fingernail or hair clippings at a cremation cemetery Wednesday, the JoongAng Daily reported.

Sailors on sea duty leave nail or hair clippings to be returned to families in case they are lost at sea. (By Jung Yeon-Je/ AFP)