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Kenyan security forces are battling to retake control of an upscale hotel and office complex in Nairobi after gunmen mounted a deadly assault on the compound on Tuesday.

At least four people were killed during what police described as a “suspected terror attack” at the complex in the Kenyan capital, a source with Salama Fakira, a private security company in the country, told CNN, cautioning that the death toll was expected to rise.

Heavy gunfire and an explosion sent people running for their lives from the compound in an affluent neighborhood on Tuesday afternoon. 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.

Hours later, the assailants were still holed up on one floor of the DusitD2 hotel inside the complex and special forces were working to “flush them out,” according to Joseph Boinnet, inspector general of Kenya’s national police service.

“Owing to the swift and targeted response of the security services, the situation has been largely contained and six out of the seven floors of the Dusit hotel have been secured,” Boinnet said at an evening news conference.

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 foyer of the hotel, where a number of guests sustained severe injuries, Boinnet said.

The number of people hurt in the attack has not yet been confirmed, but local hospitals have issued urgent appeals for blood donations.

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 University of Nairobi’s Chiromo campus were evacuated onto the street.

Social media was awash with posts from people trying to get in touch with friends still stuck inside the compound. Some said that their text messages weren’t getting through.

The Red Cross has asked families searching for loved ones to call into their 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.

“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 showing an ongoing police operation.

“We have sent in a number of teams from the anti-terror unit and CID to give support to what is going,” Kenyan police spokesman Charles Owino said. “We have to assume that this is the highest possible type incident, so we are prepared.”

The attack has drawn immediate comparisons to the Westgate mall attack in Nairobi in 2013, when Al-Shabaab extremists launched a deadly siege on a luxury shopping center, killing 67 people.

Today marks the three-year anniversary of an Al-Shabaab attack on the El Adde military base which left over 140 Kenyan soldiers dead.