British officials in New Zealand say they are waiting for news of whether any Britons have been killed in the Christchurch earthquake.
There have been reports of British casualties, but British High Commissioner Vicki Treadell warned against speculating about numbers.
Ms Treadell urged concerned relatives in the UK to call the Foreign Office helpline in London on 020 7008 8765.
A 55-strong search and rescue team is flying to New Zealand from the UK.
The team, comprising volunteers who work in various fire and rescue services across the UK, will help in the search operation.
Ms Treadell said that anyone who heard that a relative was safe should tell the authorities, so they could be discounted from any search.
At least 98 people are known to have died and police say 226 are still missing after the quake, which hit Christchurch at lunchtime on Tuesday (2351 GMT on Monday).
The Irish government has said that two of its citizens died in the earthquake. A spokesman said he had "serious concerns" about two other citizens, and concerns about a further 10.
Asked about reports of British deaths, Ms Treadell told the BBC: "That would be pure guesswork, and what we need to do is to deal with the facts so that we don't cause any greater unsettlement and grief than is necessary."
She said officials were in touch regularly with the senior police officer in charge of the mortuary where bodies were being taken.
"As and when any of those fatalities are identified as British - if we get that information - we too will be able to confirm whether it is one or more."
Later Chris Harrington, communications manager for the British High Commission, said it was not yet known if any Britons had been killed as the identification process could take several days.
He said: "We have been checking the hospitals and have a team of seven on the ground but it could be a couple of days until we know.
"The New Zealand police are handling the issue and the identification process will be careful and exacting."
New Zealand's Prime Minister, John Key, has declared a national state of emergency.
Many power and telephone lines have been knocked out, and burst water mains have flooded whole districts.
Ms Treadell has been visiting a hospital where many of the injured are being treated, and paid tribute to the doctors there.
"We have a team of staff on the ground and have been going around the triage centres in Christchurch to let people know we are here and can help them. The people have ranged from young backpackers to elderly package tourists," she said.
Speaking about the scenes in Christchurch, she said: "It is a confusing scenario. We know there are people still alive inside collapsed buildings, and those are the buildings that are taking the priority of the search."
Christchurch's airport reopened on Wednesday, and military planes were brought in to fly tourists to other cities.
Shaun Moody, the HazMat (Hazardous Materials) officer for the UK International Search and Rescue (ISAR) deployment team is among the UK search and rescue team members flying to New Zealand.
Mr Moody, who said the team works and trains together on a regular basis, added: "The teams have been deployed to Sumatra, Haiti, and also Iran, Pakistan and Turkey in previous years, so it's been going for quite some time.
"The team has been going to major disasters, and earthquakes in particular, for the last 20 years."