At least four people including two children drowned after a migrant boat sank in the English Channel near Calais on Tuesday.
French authorities said a woman and two children aged five and eight were among the victims. The body of another man was recovered earlier in the day.
Fifteen people have been rescued, but there were warnings the death toll could rise with three survivors — including two children — said to be in a "state of hypothermia".
The passengers are understood to be Iranian nationals.
One French official said it was unclear how many were on the boat when it sank at around 9.30am on Tuesday, but that there could have been up to 20 people on board.
"One person is missing, who could be an infant", said Hervé Tourmente, the official, during a press briefing at Loon-Plage, a beach between the ports of Calais and Dunkirk from where rescue services are operating.
He added that weather conditions on Tuesday "were not at all favourable" for a crossing.
A search and rescue operation was suspended as night fell.
Illegal crossings across the Channel, one of the busiest shipping lanes in the world, have become increasingly common in recent years.
Britain's Press Association agency calculated that more than 7,400 migrants had crossed by boat so far in 2020, up from 1,800 in all of 2019. Many more have been stopped by French authorities before reaching British waters.
UK Prime Minister Boris Johnson said he had offered to support the French authorities in their investigation.
He said his government "will do all we can to crack down on the ruthless criminal gangs who prey on vulnerable people by facilitating these dangerous journeys."
An investigation has been opened and entrusted to the border police, the prosecutor's office said.
Marlène Schiappa, French minister delegate for citizenship, tweeted that she had learned of the news "with great sadness" and warned that the final death toll is uncertain.
"With [Interior Minister] Gérald Darmanin and in conjunction with the British authorities, we are fully mobilised to prevent such tragedies from occurring".
The charity Save the Children said it was "awful news".
It said in a statement: "Today’s tragic news must be a wake-up call for both London and Paris to come up with a joint plan that ensures the safety of vulnerable children and families. The English Channel must not become a graveyard for children.”
Seven migrants are now confirmed to have died crossing the English Channel illegally since the start of 2020, according to figures provided by the authorities - compared to a total of four in 2019.
In recent years, attempts to cross the busy 30-kilometre shipping lane have been increasing, according to the French maritime prefecture of the Channel and the North Sea.
A total of 2,358 people were rescued and brought back to French or British coasts in 2019, compared with 586 in 2018.
European authorities arrested twelve people for smuggling migrants across the Channel in September.