CHRISTCHURCH (Reuters) - New Zealand and international rescuers held out slim hope of finding survivors three days after an earthquake struck the city of Christchurch, but with many still missing the death toll is expected to rise far above 100.

Teams from quake-prone countries such as Japan and the United States used sniffer dogs and lowered microphones into piles of rubble to probe for signs of life after a 6.3 magnitude quake struck the country's second-biggest city on Tuesday.

"We are all propelled by a desire to find our people. We aim to find them alive," said city Mayor Bob Parker on Thursday as rescue teams failed to locate any sign of life for 24 hours and a third night of searching by floodlight lay ahead.

Police say 98 people have been killed and 226 are missing, with no-one rescued from Christchurch's financial district, which bore the brunt of the damage, since Wednesday.

Instead, the ruin of the former Canterbury Television (CTV) building, which was also home to an English school for foreign students, had become a tomb, with 47 bodies pulled from the rubble and many more thought to be inside.

"Every few hours we stop and do a technical search just in case, but being realistic, based on what we've seen so far, we're not going to be seeing anyone alive coming out of that space," said fire rescue chief Jim Stuart Black.

Prime Minister John Key said many of the missing were likely to have died in the country's worth natural disaster for 80 years.

"The police have been in contact with about 200 families where there is every likelihood that that person should have returned home and they haven't," Key said.

Rescue efforts were hindered by the prospect of new collapses of damaged buildings, with most fears focused on a teetering 26-storey hotel tower as hundreds of aftershocks continued, many with magnitudes of around 4.0.

Onlookers told NZ television they could see the Hotel Grand Chancellor, around which an exclusion zone is being enforced by soldiers and police, shudder as they watched.

Police commander Dave Cliff said two infants of five and nine months were among the dead, some of whom were identified for the first time on Thursday. More were to follow on Friday.

The worst fears centered on the six-story television building, which was thought to be the deadliest single collapse with up to 120 people inside, including many missing Japanese students, caught as floors pancaked down on to one another.

But the landmark Christ Church Cathedral was also thought to have between 16 and 22 bodies inside, although searchers have listed it as too dangerous to enter.

Japanese Chief Cabinet Secretary Yukio Edano said 26 Japanese students were still missing at the CTV site, while Taiwan said one of its citizens was also thought to be inside.

(Writing by Rob Taylor; Editing by Nick Macfie)