Unidentified gunmen shot dead a senior Yemeni security official employed by the US embassy in Sanaa in a hit-and-run attack on Thursday, US and security officials said.
The murder came after a spate of attacks US interests in the Middle East, following protests last month at the US embassy in Cairo and a deadly assault on a consulate in the Libyan city of Benghazi.
Qassem Aqlan was shot dead by gunmen on a motorbike as he drove along Sittin Street, a main artery of the Yemeni capital, a Yemeni official said, adding that the assailants managed to escape.
"We're deeply saddened by the killing of Mr Aqlan, a longtime employee of the US embassy in Sanaa," State Department spokeswoman Victoria Nuland said, confirming his death.
"We condemn this vicious act in the strongest terms and extend our deepest condolences to his family and his friends at this difficult time."
Speaking in Washington, Nuland said Aqlan, who had worked at the embassy for 11 years and was a foreign service national investigator in the regional security office, was "a dedicated professional, and he will be greatly missed."
Aqlan was off-duty at the time of the attack and was shot outside the embassy, when he was with a member of his family, she added.
Staff at the Sanaa mission had an emergency meeting on Thursday to review security in the wake of the killing, which came a month after four Americans died in the attack on the US consulate in Benghazi.
"We're coordinating closely with the Yemeni authorities to investigate this attack and to help bring those responsible to justice," Nuland said.
But she stressed it was too soon to know why he had been targeted, although she ruled out earlier reports that Aqlan had been part of the team investigating the storming of US embassy in Sanaa on September 13.
"He could have been killed for reasons that had something to do with his job or reasons that had nothing to do with his job," Nuland said.
"He was head of the foreign service national investigative unit within the larger (regional security) shop, but he was not involved in this investigation."
Last month, demonstrators broke into the mission's compound, before Yemeni security forces intervened. Four protesters were killed and 34 others wounded.
Yemen's President Abdrabuh Mansur Hadi apologised to his US counterpart Barack Obama and the American people for the acts of a "mob" and ordered a probe.