Update, 9:30 p.m.
Government officials in Kenya said they have taken control of the mall and all hostages are believed to be free. Authorities also said more than 10 suspects in connection with the attacks had been arrested.
Kenya’s Interior Ministry tweeted: “Our forces are combing the mall floor by floor looking for anyone left behind. We believe all hostages have been released.”
Question still remain about the identities of the perpetrators, but Kenyan Foreign Minister Amina Mohamed told PBS Newshour Monday that "both the victims and the perpetrators came from Kenya, the United Kingdom, and the United States."
Mohamed said that two or three of the assailants may have been American young men, "about between maybe 18 and 19...of Somali origin or Arab origin, but that, you know, lived in the U.S., in Minnesota and one other place."
"At this point we have no definitive evidence of the nationalities or identities of the perpetrators,” said the State Department's Jen Psaki.
Earlier in the day, explosions roared and a massive plume of black smoke billowed out from a shopping mall in Nairobi, Kenya, on Monday as security forces moved in to try to take control of the besieged retail center.
Over the weekend, members of al Shabaab, Somalia’s al Qaeda-linked rebel group, dressed in black clothing and ski masks, stormed the Westgate shopping center, went on a shooting spree and killed at least 62 people. The standoff lasted three days.
Early Monday at least 15 attackers and 15 hostages were still believed to be inside the mall. At least 175 had been injured, including five Americans.
President Obama, at the beginning of a bilateral meeting in New York with President Goodluck Jonathan of Nigeria, said he recently called Kenyan president Uhuru Kenyatta to express condolences.
"This attack underscores the degree to which all of us in the international community have to stand against the kind of senseless violence that these kinds of groups represent. And the United States will continue to work with the entire continent of Africa and around the world to make sure that we are dismantling these networks of destruction," said Obama.
U.S. Secretary of State John Kerry called the attackers “ruthless and valueless terrorists,” during a conference at the United Nations on Monday. He added the U.S. would stand in support with Kenya in the days ahead.
“Whether it’s in downtown Manhattan or a mall in Nairobi or anywhere else in the world, all of us have a responsibility to remain vigilant…This tragedy is a reminder, a terrible reminder to all of us, that we all share a stake in one another,” said Kerry.
The attack is the deadliest act of terrorism in Kenya since the bombing of the U.S. embassy in 1998, in which more than 200 people were killed.
Deputy National Security Adviser Ben Rhodes aboard Air Force One said Monday that while there are concerns about al Shabaab’s efforts to recruit Americans, there was no confirmation any Americans were involved in the latest attack.
Some of the survivors, including Radio Africa journalist Kamal Kaur took to Twitter to detail their horrific ordeal, detailing how she saw her six-month pregnant coworker and friend, Ruhila Adatia, killed.
“Images of the day keep flashing in front of my eyes. Dead bodies, injured people, wailing crying, scared,” she wrote in one post.
“My son almost had his head blown off. Missed by an inch. Hit wall. Bounced & killed the little boy next to him," Kaur wrote in another.
NBC's Catherine Chomiak and Richard Esposito contributed to this report.