At least two Americans are among the dead from the twin terror attacks in Brussels earlier this week, a senior U.S. official told Fox News on Friday.

Authorities have not released the identities of the U.S. citizens who were among the 31 people killed in coordinated bombings Tuesday at Zaventem Airport and a metro station. But Secretary of State John Kerry acknowledged the deaths of Americans after meeting with Belgian Prime Minister Charles Michel Friday.

"The United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks," Kerry said after meeting Michel in Brussels.

"The United States is praying and grieving with you for the loved ones of those cruelly taken from us, including Americans, and for the many who were injured in these despicable attacks."

- Secretary of State John Kerry

A senior official said the families of two Americans had been informed of their deaths. The official, who spoke on condition of anonymity, did not have further details.

Relatives of siblings Sascha and Alexander Pinczowski, Dutch nationals living in New York known to have been at the aiport, told CBS News they had been informed their loved ones were among the dead.

The Islamic State terror group has claimed credit for the attacks, which were carried out by as many as five bombers, three of which are believed to have died and two who are being sought.

About a dozen Americans were known to be injured in the attacks, and several stateside families had not been able to contact relatives that were known to be at the airport, but Friday's development marked the first confirmation of American deaths.

The State Department said among those still unaccounted for were two "official Americans," meaning they were U.S. govement employees or their family members. That description could include both State Department employees, military employees or others, he said.

Also among the U.S. citizens still unaccounted for were Justin and Stephanie Shults, originally from Teessee and Kentucky but now living in Belgium. Adding to their relatives' anxiety, the Shults were wrongly said to have been located on Wednesday.

Justin's brother, Levi Sutton, said a State Department official told his mother the couple had been found, but that an hour later a social worker called the mother to say the information was incorrect.

Counted among the Americans confirmed injured in the bombings were three missionaries from the Utah-based Church of Jesus Christ of Latter-day Saints, as well as a U.S. Air Force airman and four members of his family.

Kerry, who arrived at the still-closed airport Friday for a brief unscheduled stop on his way home from Moscow, pledged U.S. support for Belgium in the wake of the attacks, even as President Obama was criticized for appearing at a baseball game in Cuba and dancing the tango in Argentina during a previously scheduled tour earlier this week.

"We - all of us representing countless nationalities - have a message for those who inspired or carried out the attacks here or in Paris, or Ankara, or Tunis, or San Beardino, or elsewhere: We will not be intimidated," Kerry said. "We will not be deterred. We will come back with greater resolve - with greater strength - and we will not rest until we have eliminated your nihilistic beliefs and cowardice from the face of the Earth."

Michel thanked Kerry for his visit, calling it a powerful message of solidarity. "It is very important for us today to receive your support," he said. He offered condolences for the American victims and vowed to step up counter-terrorism cooperation with the U.S. and others.

Kerry said he offered the prayers of the American people for "these people who have suffered inconceivable losses."

"Those whose lives were to apart this week were not combatants in any conflict," the secretary said.

The Belgian Embassy, not long after Kerry's arrival, sent a Twitter message calling his stop here an example of "the solidarity of the American people which goes right to our heart."

The Associated Press contributed to this report