The United States condemned the attack by an al Qaeda-linked militia on the United Nations office in Somalia Wednesday, calling the terrorist tactics used by attackers “despicable” and “repugnant” and pledging to remain a “steadfast partner” of the Somali government.

A gun battle inside the U.N. Development Program headquarters in the Somali capital of Mogadishu lasted more than an hour Wednesday and killed at least 15 people, according to reports of eye-witness accounts. The attack began when a suicide bomber blew up a car at its entrance, creating a distraction that allowed four gunmen to storm security guards and enter the office.

Al-Shabab, an al Qaeda-linked militia that opposes the Somali government, claimed responsibility for the attack on its Twitter account.

“The United States condemns in the strongest terms al Shabab’s despicable attack on the United Nations in Somalia today,” U.S. National Security Council spokeswoman Caitlin Hayden said in a statement. “The attack targeted people and organizations working in partnership with the Somali government to provide health services, clean water, sanitation, education, and economic opportunity – people striving to build a better future for the Somali people.

Hayden also commended the “brave and swift response” of the Somali security forces who rushed in and returned fire, saving the lives of countless others working for the U.N. or alongside it. She also sent the United States’ “deepest condolences to the families and loved ones of the victims of this heinous act.”

Somali Interior Minister Abdikarim Hussein Gulen said all seven attackers died, but not before they killed four foreign security workers and four local guards.

Two of those who died worked for South African state weapons firm Denel, the company has acknowledged. The U.K. Foreign and Commonwealth Office said it was pursuing reports that three British nationals are among those who died.

The attack comes after a period of relative stability in Somalia. The U.N. has only recently expanded its operations in Mogadishu after years when its Somalia mission was based in neighboring Kenya because of security fears, according to the BBC. Earlier this month, the new Somali President, Hassan Sheikh Mohamud, visited London to much fanfare, and the U.N. declared the end of a tumultuous eight-year political transition in the nation.

The U.S., in its statement, said al Shabab and its penchant for violence won’t stand in the way of U.N. efforts to ease the suffering of the Somali people by distributing food and medical aid during one of the region’s worst droughts.

“The United States remains a steadfast partner of the Somali people and their government as they work to build a safer, more prosperous future,” Hayden said. “We reiterate our strong support for the important work of the United Nations toward reaching that goal. Those seeking to derail Somalia’s progress will not succeed.”