A Dutch brother and sister, a Chinese national and a British man were on Friday confirmed as being among the 31 people killed in Tuesday's bomb attacks on Brussels airport and the metro system.

US Secretary of State John Kerry also said Friday that a number of Americans died in the attacks, which were claimed by the Islamic State jihadi group.

Kerry did not give a figure but a US official separately said two US citizens had been confirmed dead.

As forensic experts continued to sift through evidence, the Dutch foreign ministry announced that a brother and sister who lived in the United States and a woman from the central Dutch city of Deventer were among the victims.

"Our thoughts are with their relatives and friends,"Dutch Foreign Minister Bert Koenders said in a statement issued in The Hague.

"It's awful that these people were killed by a random act of terror."

Dutch public newscaster NOS later named the brother and sister as Alexander Pinczowski and Sascha Pinczowski, both 21, who lived in New York.

A Dutch newspaper said they were on the telephone to a relative when the bombs went off and the line went dead.

NOS named the other Dutch victim as Elita Weah, 41, who was on her way to her stepfather's funeral in the US.

- 'Devastating news' -

Britain's Foreign Office confirmed that computer programmer David Dixon, 51, from Hartlepool in northern England, but who lived in Brussels, had died in the metro attack.

"We can confirm David Dixon lost his life in the attacks which took place in Brussels on Tuesday 22 March 2016," the ministry said in a statement.

"We know of seven British nationals who were injured in the attacks -- three are still being treated in hospital."

His family issued a statement calling the news "terrible and devastating".

Dixon texted his aunt after the airport blasts to say he was safe, but happened to be on the metro system when a suicide bomber blew himself up.

A Chinese national was also among those killed, the Chinese embassy in Belgium confirmed Friday.

"We express deep condolences over the death of our Chinese compatriot and strong condemnation on the criminal act of the terrorists," said the Chinese embassy on its website.

The family of 21-year-old Bart Migom, who was travelling to see his American girlfriend, had declared him missing, and the Belgian college where he was a student on Friday said he was one of the victims.

Along with the 31 dead, 300 were injured, 61 critically.

Identification is proving slow, complicated by the violence of the explosions and because many of the victims were foreigners, police told RTBF television.

- Facebook appeal -

Around 40 nationalities are thought to be among the dead and wounded.

Their diverse backgrounds reflect the cosmopolitan nature of Brussels, Europe's symbolic capital.

Some 30 specialists, including the seven permanent Disaster Victim Identification (DVI) team experts, are working to identify the bodies or remains of victims recovered from the attack scenes.

A Facebook page where worried relatives, friends and colleagues can post notices of the missing has been set up. Pictures already uploaded show men and women, young and old, from Belgium and across the globe."

Ecuadoran President Rafael Correa announced Thursday that Jimmy Montenegro, 37, from the northern city of Ibarra, was in a "very serious" condition after being caught in the metro blast.

Dixon's identification brings the number of officially named fatalities to four.

Adelma Marina Tapia Ruiz, a 37-year-old Peruvian woman who lived in Belgium, was killed in the airport blasts, the foreign ministry in Lima confirmed.

Another victim was Belgian civil servant Olivier Delespesse, according to his employer.

He was killed in the metro attack, local media reported, along with 20-year-old Belgian law student Leopold Hecht, who was named by his university.