South Korean divers have made several trips to the wreckage
South Korean divers have reached a warship that sank in mysterious circumstances on Friday after a blast.
The divers tapped on the stern of the Cheonan, where some of the 46 missing crew members are believed to have been trapped, but heard no response.
The vessel sank close to the sea border with North Korea; South Korea says it is open-minded about the blast's cause.
But the country's defence minister said it could have been caused by a North Korean mine.
At the weekend, rescue officials said an explosion broke the ship into two.
Fifty-eight crewmen were saved but the others are missing. The authorities said they hoped they were trapped in underwater air pockets in the wreckage.
A spokesman for the Joint Chiefs of Staff, Lee Ki-shik, said divers had reached the sunken vessel.
Relatives of the missing have been demanding answers from officials
"There was no response when military divers knocked on the hull of the ship's rear," he told journalists.
"Divers went underwater several times, working in a very limited time of about seven to eight minutes each time, but failed to find any survivors."
Earlier attempts to reach the vessel had been hampered by strong currents and murky waters.
Yonhap news agency quoted officials as saying that parts of the ship were able to shut out water for a maximum of 69 hours.
South Korea President Lee Myung-bak urged rescuers not to give up hope of finding survivors and to investigate all possible causes of the sinking.
"Look into the causes of the incident thoroughly and leave no single piece of doubt behind," the AFP news agency quoted him as saying.
Yonhap said 24 military vessels, with air support, were involved with the search. A US rescue vessel, with more than a dozen divers on board, has also joined the operation.
Relatives of those missing have angrily demanded progress and answers from officials. "We are running out of time," one woman told reporters.
The 1,200-tonne Cheonan naval patrol vessel sank near the disputed maritime border with North Korea - the Northern Limit Line.
South Korea recognises the line, drawn unilaterally by the US-led United Nations Command to demarcate the sea border at the end of the Korean War.
But it has never been accepted by North Korea, and the area has been the scene of deadly clashes between the navies of the two Koreas in the past.
South Korean Defence Minister Kim Tae-young said the explosion could have been caused by an old North Korean wartime sea mine which drifted into southern waters.
"Though many mines were removed, it must have been impossible to retrieve them all," AFP quoted him as saying.
"Or we have to see whether North Korea has intentionally set a mine adrift," he added.
North Korea has made no official comment on the incident.