Waves up to 20ft high triggered by an earthquake earlier this week have claimed nearly 150 lives so far in Samoa, American Samoa and Tonga, and led to a huge clean-up operation.
As another quake struck the region on Friday, British consular officials were trying to trace "a number" - reportedly 16 - of British nationals thought to have been in the area when the tsunami struck.
A Foreign Office spokesman said: "Our consular team on the ground are in contact with the local authorities and are assisting any British nationals that have been affected by this terrible tragedy.
"We have received reports of a number of British nationals on Samoa or in the surrounding region that we have not yet been in contact with. Communications on the island remain severely disrupted. Phone lines are down and many roads impassable.
"Because the situation is developing rapidly and we are receiving updates all the time, we cannot be drawn into speculating on numbers."
The two-year-old son of a British couple swept out to sea is feared to be among the existing tally of tsunami victims.
His father was treated in hospital after being injured on the beach where the wave struck.
A spokesman for the British High Commission in New Zealand said he was taken to hospital in the Samoan capital of Apia.
"He was brought to Apia from the south coast and admitted to hospital," he said. "I don't have any information about his injuries, or whether he is still in hospital, but I understood he was in a stable condition."
The husband and wife, originally from Britain, now live in the Auckland area of New Zealand, sources said.
A Foreign Office spokeswoman said the boy was "missing, presumed dead".
Tuesday's tsunami happened after an earthquake, with a magnitude of between eight and 8.3, struck about 120 miles from American Samoa, a US territory.
Thousands of people were left homeless as entire communities were destroyed.
Meanwhile Scottish volunteers from the civilian International Rescue Corps, based in Grangemouth, are flying out to the Indonesian island of Sumatra where earthquakes have killed more than 1,100 people and left many more homeless.
The group were asked to go out by the Indonesian government and will take specialist equipment including listening devices and thermal cameras which can detect trapped survivors.
They will be joined by a 60-strong team of firefighters including personnel from the West Midlands, Hampshire, Leicestershire, Cheshire, Greater Manchester, Lincolnshire, Lancashire, Kent, Essex, West Sussex and Wales.
A plane carrying firefighters and specialist equipment was flying out from Gatwick for the Sumatran city of Padang, where hundreds of buildings collapsed after two massive tremors.
The flight was organised by officials from the UK’s Department for International Development (DFID) who will join local teams on the ground trying to save as many people as possible.
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