PARIS, France (AP) — At least four people, including two young children, died Tuesday when a boat carrying at least 19 migrants capsized off the coast of France while trying to cross the English Channel to Britain, French authorities said.

Fifteen migrants have been saved so far and rescue and search operations were still under way, according to the regional administration for the Nord region. It said in a statement that the dead were a five-year-old and an eight-year-old, an adult woman and an adult man, and stressed that the toll so far is provisional pending further searches in the area.

Aid groups decried the deaths and called for more government help for struggling migrants, while British and French authorities expressed their condolences.

Such crossings have become increasingly common in recent years, despite political uproar in Britain and stepped-up police efforts to stop them, but deaths are rare. French authorities reported four migrant deaths in total in small boats crossing the Channel over all of 2019.

On Tuesday, a sailboat alerted authorities to a migrant boat in distress off the coast of Dunkirk, and France mobilised five vessels and a Belgian helicopter nearby to help with the rescue, according to the regional French maritime agency. It had said earlier that 18 people were rescued and were receiving treatment in hospitals in Calais and Dunkirk. The reason for the different numbers of rescued migrants was not explained.

The Dunkirk prosecutor launched an investigation into the cause of the sinking, which occurred on a day when both British and French weather authorities had warned of gusty winds in the area.

Despite joint police efforts on both sides of the Channel, migrants have long used northern France as a launching point to sneak into Britain, and the issue has long strained relations between the neighbours.

Britain’s Press Association news agency calculates that more 7,400 migrants have crossed the Channel to the UK by boat so far this year, up from about 1,800 in all of 2019. French maritime officials have rescued hundreds more in routine operations in the Channel, known for high winds, strong currents and heavy maritime traffic.

British Home Secretary Priti Patel said she was “truly saddened to learn of the tragic loss of life in French waters this morning” and offered UK help with the search and investigation.

“My thoughts and prayers are with their families and loved ones at this time,” Patel said in a statement. “This tragic news highlights the dangers that come with crossing the Channel and I will do everything I can to stop callous criminals exploiting vulnerable people.”

France’s minister for citizenship issues, Marlene Schiappa, tweeted “great sadness” at the news. She said 19 people had been confirmed aboard the boat, but the overall toll “is serious, and still uncertain”.

Aid group Channel Rescue said in a statement that “This is a burgeoning humanitarian crisis …. The government must urgently ensure safe and legal passage for those seeking safety.”

Clare Mosely of migrant support group Care4Calais said: “This unnecessary loss of life has to stop. No one should ever feel they have to get into a fragile craft and risk their lives crossing the Channel, least of all vulnerable children.”

She called for the incident to be a “wake-up call” for those in power in the UK and France.