Two New York City-based siblings, who were visiting Brussels, died in Tuesday's terror attacks, a family spokesman confirmed to NBC 4 New York on Friday.

Sascha and Alexander Pinczowski had traveled to Brussels Airport on Tuesday and were about to check in for a flight back to New York around the time the two blasts went off, officials said.

Relatives told a Dutch newspaper that the siblings called them around the time of the attack and that they heard a blast and shattering glass before the line went dead. They said they have been unable to reach the siblings since then.

"We received confirmation this morning from Belgian Authorities and the Dutch Embassy of the positive identification of the remains of Alexander and Sascha," James Cain, a spokesman for the siblings' family, said in a statement. "We are grateful to have closure on this tragic situation, and are thankful for the loving support, thoughts and prayers from all."

He added the family is in the process of making arrangements.

Cain, the former U.S. Ambassador to Denmark and father of Alexander Pinczowski's fiancée, told NBC News in an email that the Pinczowski siblings are Dutch citizens, not American, as has been previously reported. They had lived in New York and intended to become American citizens, he said.

Alexander Pinczowski had traveled to Holland to work on a craft-related business that he and Cameron were going to start together, Cain said.

The couple met six years ago while taking summer courses in Durham, North Carolina. They hadn't set a wedding date but had planned to marry within the year, Cain said.

He called Alexander "intimidatingly smart, a brilliant young man."

Sascha Pinczowski, 26, was a 2015 graduate of Marymount Manhattan College in New York with a degree in business. She spent last summer as an intern at a catering company, Shiraz Events.

The president of Shiraz Events said the business is "heartbroken" to learn of the siblings' death.

"We remember her as a warm and lively presence during her too brief time with us," said Shai Tertner. "Her maturity, motivation, humor and professionalism were marked characteristics in someone of her young age and we felt that she had a great future in our industry. We will miss her tremendously."

Mayor Bill de Blasio tweeted condolences Friday morning.

"Two of our own lost in the #brusselsattack. In memory of the Pinczowski siblings, we will continue to stand up to terror at every turn," the mayor said.

Gov. Andrew Cuomo called the terrorists in the attacks that killed the siblings cowards in a statement Friday.

"On behalf of all New Yorkers, I extend our deepest prayers and condolences to the Pinczowski family, as well as all those who lost loved ones in Tuesday's heartbreaking attacks," Cuomo said.

At least 31 people died and more than 270 were injured in the explosions at the airport and at a metro station. ISIS has claimed responsibility.

Earlier on Friday, a senior U.S. official told NBC News that two Americans had been confirmed killed in the attacks, without giving their identities.

Two other Americans, Justin Shults and his wife Stephanie, have also been missing since the attacks, according to family members. The U.S. State Department said it is aware of 12 Americans injured in the blasts but did not provide their names or conditions.

British officials confirmed Friday that a U.K. citizen died in the attacks. David Dixon, a computer programmer living in Brussels, was killed in the bombing on the subway, the officials said.

A Chinese national is also reported to have been killed, according to the Chinese Embassy in Belgium. He was identified only by his surname, Deng, and no other details were released.