Rescue workers are searching for both survivors and bodies
British consular officials are trying to trace "a number" of UK nationals thought to have been in the South Pacific region when a tsunami struck.
Rescue workers on Samoa, America Samoa and Tonga are searching for survivors.
The deadly waves, which killed at least 149 people, were triggered by an 8.3-magnitude earthquake on Tuesday.
The Foreign Office said it had received reports of "British nationals on Samoa or in the surrounding region that we have not yet been in contact with".
A 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.
"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."
A British couple's two-year-old son who was swept out to sea off Samoa is feared to be among those killed by the tsunami.
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.
It is thought that the husband and wife, originally from Britain, now live in the Auckland area of New Zealand.
A Foreign Office spokeswoman said the boy was "missing, presumed dead".