There is "genuine concern" Premier League footballer Emiliano Sala was on a flight missing off Alderney.
It is feared the Argentine striker was one of two people on board the Piper Malibu, which disappeared near the Casquets lighthouse on Monday night.
Cardiff City, which signed the 28-year-old from French club Nantes on Saturday, said it was "seeking clarification" as to what happened.
A helicopter and lifeboat crews joined a search for the missing plane.
HM Coastguard said the disappearance was not in the UK search and rescue area, but it had sent two helicopters to help.
A spokeswoman said they were assisting Guernsey Coastguard with a search for a light aircraft off Alderney: "HM Coastguard helicopters from Solent and Newquay have been helping to search overnight with nothing found."
The search and rescue operation had to deal with worsening conditions as they scoured the area on Monday night.
The Piper Malibu plane is reported to have lost contact near the Casquets lighthouse.
The Met Office said at the time the aircraft went missing "there were some showers around, but nothing too intense".
"Wind speeds were not too bad - average speeds were around 15 to 20mph," it added.
As the search continued into the evening though, it became wetter and windier.
The Casquets are infamous among mariners as the site of many shipwrecks
A lighthouse was first built on the Casquets rocks, infamous among mariners as the site of many shipwrecks, 13km north-west of Alderney in 1724.
HM Coastguard said the helicopter from Newquay has now returned to base to refuel and rest the crew.