South Korean sailors who survived the sinking of their naval ship last month spoke about their ordeal Wednesday and described hearing a huge blast before the vessel broke apart near the North Korean border.

The explosion at the stern of the Cheonan on March 26 shut down its engine, wiped out power and caused the 1,100-tonne ship to sink a little over three hours later.

Survivors of the sunken South Korean ship attend a news conference near Seoul on Wednesday. (Jung Yeon-je/Reuters)

In a news conference at a military hospital near Seoul, some of the 58 sailors rescued from the vessel told reporters there were no unusual signs before the explosion. The men — dressed in their hospital gowns, some near tears — said there was no smell of gunpowder after the blast, only oil.

Sailors said the blast felt like it came from outside the ship, but did not comment on speculation of possible North Korean involvement. The Cheonan, which had a crew of 104 sailors, had been on a routine patrol near South Korea's Baengnyeong Island, just south of the disputed maritime border.

Senior Chief Petty Officer Kim Soo-gil said he heard a booming noise, then the sound of water. He followed the trail of moonlight reflecting on the water flooding into the boat to reach the boat's deck from his bunk inside.

"There's a tearing noise if a ship hits a rock, and it would have shaken if it was caught in sand," he said. "It seemed to have been a shock from the outside rather than those two reasons."

Fellow officer Oh Seong-tak, who was in his bunk on the vessel's lowest level at the time of the blast, said he groped around him to feel his way out, and then realized the door was at his feet.

"I heard a loud boom, and felt my body being instantly lifted up in the air. ... The vessel tilted at a 90-degree angle immediately after the explosion," Oh said. "The noise was so loud that my ears hurt."

No cause for the disaster has been determined. South Korean officials have said they will examine every scenario, including the possibility that the ship may have hit a floating mine or been struck by a North Korean torpedo.

Speculation has been intense in South Korea. The government, however, has urged calm and said the cause can only be determined after remains of the ship are salvaged and thoroughly examined.

"Instead of saying this or that, we must be patient and wait," President Lee Myung-bak said Wednesday.

Lee said he asked U.S. President Barack Obama to send military and civilian experts to help investigate the incident.

Efforts to recover the ship began earlier this week.

Divers recovered the body of one crewman Saturday and a second on Wednesday.

The 44 sailors still missing have not been declared dead. However, rescue operations were called off Saturday at the request of family members after a navy diver lost consciousness underwater and died while looking for survivors.

With files from The Associated Press