US officials on Wednesday sought to confirm the whereabouts of Americans missing in Brussels following Tuesday's suicide bombings and Atlanta's airport was briefly evacuated because of a suspicious package as travellers remained on edge.

Public areas of the domestic terminal at Atlanta's Hartsfield-Jackson International Airport, the nation's busiest by passenger volume, were briefly evacuated on Wednesday morning while police investigated a suspicious package, airport officials said on Twitter. The site was quickly cleared and airport operations resumed.

The evacuation came a day after at least 31 people were killed and 260 wounded in the attacks on Brussels' airport and subway. US officials said on Tuesday that eight Americans, including three Mormon missionaries and a US Air Force airman and four members of his family, were injured in the bombings.

State Department spokesman Mark Toner told CNN on Wednesday that it now appeared that about a dozen Americans were injured. He could not confirm whether any US citizens had been killed.

‘We're still getting information. We're still trying to seek out the whereabouts of American citizens,’ Toner said. ‘Obviously, Brussels on any given day is chock full of American citizens.’

Apart from the eight Americans confirmed as wounded on Tuesday, US media reported on Wednesday that relatives of at least four other Americans who had been traveling in Belgium are still trying to track them down.

Husband and wife Justin and Stephanie Shults, originally from Tennessee and Kentucky, respectively, but now living in Belgium, also were missing after having dropped off a relative at the airport shortly before the blasts, Justin Shults' brother, Levi Sutton, said in an electronic message.

‘We haven't been able to contact them going on 30 hours,’ Sutton said. ‘Stephanie's mom is fine but she was separated from Justin and Stephanie.’

Sister and brother Sascha and Alexander Pinczowski, who had been living in New York, remain unaccounted for, the New York Daily News reported. The Pinczowskis' citizenship was unclear. (http://nydn.us/22uREmM)

A woman who identified herself on social media as Alexander Pinczowski's girlfriend said she had been unable to contact him since Tuesday morning.

Major transportation hubs were on high alert following the attacks, although Homeland Security Secretary Jeh Johnson said there were no known specific and credible threats to the United States as of Tuesday.

Parts of Denver International Airport were briefly evacuated Tuesday in what turned out to be a false alarm over a suspicious package.