Protesters have stormed the grounds of the United States embassy in the Yemeni capital, Sanaa.
They were demonstrating against an anti-Islam film made in the US, which has provoked anger around the Muslim world.
On Tuesday, the US ambassador to Libya was among several people killed in a fire started after the US consulate in Benghazi was stormed.
There have also been clashes in the Egyptian capital, Cairo.
Witnesses in Sanaa said a number of vehicles had been set on fire in the compound of the embassy.
Police opened fire in an attempt to hold back the crowds, but they managed to gain access to the building.
Earlier on Thursday, US officials said they were investigating whether the attack in Libya was planned, citing suspicions that a militant jihadist group may have co-ordinated the violence.