The terrorist al-Shabab group says its gunmen are still alive and holding hostages in the Nairobi shopping mall they seized four days ago.
The al-Qaida-linked organisation made the claims after Kenyan government assurances of success in the fourth-day of the stand off.
Al-Shabab said the hostages "are still alive, looking quite disconcerted but, nevertheless, alive."
A security expert with contacts inside said there were at least 30 hostages when the assault began on Saturday and at least 10 were believed to be still inside.
Sporadic gunfire could be heard from the building on Tuesday as government forces pressed on inside.
Americans and a British woman were among the terrorists who have killed more than 60 people, Kenya's foreign minister said. Amina Mohamed said the attackers included "two or three Americans" and "one Brit". She told the PBS NewsHour programme that the Americans were 18 to 19 years old, of Somali or Arab origin and lived "in Minnesota and one other place" in the US. The Briton was a woman who has "done this many times before", she said.
US officials said they were looking into whether any Americans were involved. State Department spokeswoman Jen Psaki said the department had "no definitive evidence of the nationalities or the identities" of the attackers.
The security expert said many hostages had been freed or escaped in the previous 24-36 hours, including some who were in hiding. But there were at least 30 hostages when the assault by al-Shabab militants began on Saturday, he said, and "it's clear" that Kenyan security officials "haven't cleared the building fully".
Kenya's interior minister Joseph Ole Lenku said the evacuation of hostages had gone "very, very well" and that Kenyan officials were "very certain" that few if any hostages were left in the building.
But with the mall cordoned off and under heavy security it was not possible to independently verify the assertions. Similar claims of a quick resolution were made by Kenyan officials on Sunday and the siege continued. Authorities have also not provided any details on how many hostages were freed or how many still remain captive.
Al-Shabab, whose name means "The Youth" in Arabic, said the mall attack was in retribution for Kenyan forces' 2011 push into neighbouring Somalia. It was the deadliest terrorist attack in Kenya since the 1998 al Qaida truck bombing of the US embassy in Nairobi, which killed more than 200 people.
An extremist Islamic terrorist force that grew out of the anarchy that crippled Somalia after warlords ousted a long-time dictator in 1991, al-Shabab is estimated to have several thousand fighters, including a few hundred foreigners, among them militants from the Middle East with experience in the Iraq and Afghanistan conflicts. Others are young, raw recruits from Somali communities in the United States and Europe.