(CNN) Kenyan security forces have secured the buildings in Nairobi where gunmen mounted a deadly attack Tuesday, a government official said.

"The security teams have evacuated scores of Kenyans and other nationalities from the (hotel and office) buildings," Ministry of Interior and Coordination of National Cabinet Secretary Fred Matiang'i said in a statement. "We are now in the final stages of mopping up the area and securing evidence and documenting the consequences of these unfortunate events." At least 11 people have been killed during what police described as a "suspected terror attack" at the complex in the Kenyan capital, said a source at the hotel assisting with security. The source called the death toll "conservative" and expected it to rise: "I saw six dead on the footpath exit over the river and five more at the secret garden cafe." Heavy gunfire and an explosion sent people running for their lives Tuesday afternoon at the compound in an affluent neighborhood. Footage from the scene showed armed officers escorting office workers and injured people to safety as the sound of shots rang out. Cars in the compound could be seen aflame as a helicopter flew overhead. A woman is escorted from the Riverside Drive complex in Nairobi. Read More The coordinated attack began at a bank inside the complex, with an explosion targeting three vehicles in the parking lot, followed by a suicide blast in the hotel foyer, where guests sustained severe injuries, Joseph Boinnet, inspector general of Kenya's national police service, told reporters. The number of people hurt in the attack has not yet been confirmed, but local hospitals have issued urgent appeals for blood donations. Security forces responding to the scene. The Somali Islamist militant group Al-Shabaab claimed responsibility for the attack, according to SITE Intelligence Group. Anti-terror units, other law enforcement agencies and ambulances rushed to the scene on Riverside Drive in Nairobi's Westlands neighborhood as people with bloodied clothes could be seen fleeing the complex. Nearby, students from the University of Nairobi's Chiromo campus were evacuated onto the street. Social media featured posts from people trying to get in touch with friends still stuck inside the compound. Some said their text messages weren't getting through. The Red Cross has asked families searching for loved ones to call into its dedicated tracing hotline for help. Police asked that residents refrain from sharing unconfirmed news, or images of the ongoing security operation on social media, cautioning about misinformation circulating online. Photos: Nairobi hotel attack Security forces help people flee the scene of an attack at a hotel and office complex in Nairobi, Kenya, on Tuesday, January 15. Hide Caption 1 of 28 Photos: Nairobi hotel attack Civilians flee as security forces aim their weapons at the buildings of a hotel complex in Nairobi during a deadly attack on Tuesday. Hide Caption 2 of 28 Photos: Nairobi hotel attack A member of the security forces is seen reflected in a window as he looks at bullet holes. Hide Caption 3 of 28 Photos: Nairobi hotel attack A woman is reunited with her family after her evacuation. Hide Caption 4 of 28 Photos: Nairobi hotel attack Civilians flee the scene during an attack on a hotel complex in Nairobi. Hide Caption 5 of 28 Photos: Nairobi hotel attack Damages and blown out windows are seen in the hotel complex after the attack. Hide Caption 6 of 28 Photos: Nairobi hotel attack Special forces protect people at the scene of an explosion at the DusitD2 Hotel complex. Hide Caption 7 of 28 Photos: Nairobi hotel attack Kenyan security forces aim their weapons as they move through the hotel complex. Hide Caption 8 of 28 Photos: Nairobi hotel attack Security forces arrive at the scene of the attack. Hide Caption 9 of 28 Photos: Nairobi hotel attack An injured man is from helped from scene. Hide Caption 10 of 28 Photos: Nairobi hotel attack People take cover after hearing gunfire coming from the hotel. Hide Caption 11 of 28 Photos: Nairobi hotel attack Burned cars are seen at the site of an explosion at the DusitD2 Hotel. Hide Caption 12 of 28 Photos: Nairobi hotel attack A fleeing civilian throws her handbag before climbing out of a hotel window. Hide Caption 13 of 28 Photos: Nairobi hotel attack A woman reacts after being rescued from the hotel in the Kenyan capital. Hide Caption 14 of 28 Photos: Nairobi hotel attack A woman collapses after being rescued. Hide Caption 15 of 28 Photos: Nairobi hotel attack People run for cover after being rescued from the hotel. Hide Caption 16 of 28 Photos: Nairobi hotel attack Journalists carry a victim from the scene of an explosion. Hide Caption 17 of 28 Photos: Nairobi hotel attack Security forces surround a hallway in which an unexploded grenade lies. Hide Caption 18 of 28 Photos: Nairobi hotel attack An unexploded grenade lies in a hallway. Hide Caption 19 of 28 Photos: Nairobi hotel attack People put their hands up after being evacuated. Hide Caption 20 of 28 Photos: Nairobi hotel attack A woman is evacuated from the scene. Hide Caption 21 of 28 Photos: Nairobi hotel attack Civilians flee the scene. Hide Caption 22 of 28 Photos: Nairobi hotel attack Security forces gather outside the hotel. Hide Caption 23 of 28 Photos: Nairobi hotel attack A woman is led away from the hotel attack. Hide Caption 24 of 28 Photos: Nairobi hotel attack Security forces on alert near the scene of the attack. Hide Caption 25 of 28 Photos: Nairobi hotel attack Three women flee the hotel complex. Hide Caption 26 of 28 Photos: Nairobi hotel attack Security forces work at the scene of the attack. Hide Caption 27 of 28 Photos: Nairobi hotel attack A plainclothes police officer directs traffic on a highway near the scene of the attack. Hide Caption 28 of 28 "As we were leaving, there were gunshots all over the place," Evans Ng'ong'a, who was inside the complex, told CNN. "Attackers jumped over the fence and started shooting after the explosion." Ng'ong'a shared videos and photos on Twitter of an ongoing police operation. US Ambassador to Kenya Bob Godec condemned the attacks in a series of tweets and offered US assistance. He confirmed that all embassy personnel are safe, although it's unclear at this point if any US citizens are involved. UN Secretary-General António Guterres also condemned the attack and expressed "his solidarity with the people and government of Kenya," according to a statement . People run for cover after being rescued from the compound. The attack drew immediate comparisons to the 2013 Westgate mall attack in Nairobi when Al-Shabaab extremists killed 67 people at the luxury shopping center. Tuesday marks the three-year anniversary of an Al-Shabaab attack on the El Adde military base, which left more than 140 Kenyan soldiers dead.