SANAA, Yemen (AP) — A Yemeni university professor and secular thinker was killed on Saturday in a drive-by shooting in southern Yemen, local officials said.

Khalid al-Hameidi was a known critic of Islamic extremists, at a time when dissent has become dangerous amid Yemen’s yearslong civil war.

No group immediately claimed responsibility for the shooting.

The officials said that the gunmen are believed to have been either members of al-Qaida in the Arabian Peninsula, which the U.S. considers the world’s most dangerous offshoot of the terror network, or an affiliate of the Islamic State group.

The two militant groups regularly strike security and military targets in drive-by shootings, suicide bombings and other attacks. They also target those who speak out against the militants or their ideology.

The officials said two gunmen on a motorcycle shot al-Hameidi in the city of Dhale, where he was dean of the university's education faculty.

The officials spoke on condition of anonymity because they were not authorized to brief the media.

Known for his secular thinking, al-Hameidi was a harsh critic of religious extremism, and encouraged his students to organize and take part in mixed-gender cultural and artistic activities in Dhale University.

Thousands took part in al-Hameidi’s funeral Saturday in a show of solidarity against extremist groups, the officials said.

Yemen plunged into chaos and civil war when Iranian-backed Houthi rebels took over the capital, Sanaa, in 2014 from the internationally recognized government. A Saudi-led coalition allied with the government intervened to fight the Houthis the following year.

Militias established and funded by the United Arab Emirates, which intervened alongside its ally Saudi Arabia, mostly control Dhale province.

The war in Yemen has led to what the U.N. has called the world’s worst humanitarian crisis.