The United States faces increased threats from extremist groups at home and abroad, as evidenced by the hostage-taking at a Texas synagogue last month and threats to plant bombs at many colleges and universities that historically have had a majority of African American students, the US Department of Homeland Security said Monday, the Voice of America reported.

The warning came after some schools in the US canceled classes last week and issued self-exclusion orders. Investigators were ultimately unable to find any explosives.

"Threats directed at Historically Black Colleges and Universities (HBCUs) and other colleges and universities, Jewish facilities, and churches cause concern and may inspire extremist threat actors to mobilize to violence," the Homeland Security Department said in a bulletin.

Last month, an armed man took four people hostage at Beth Israel Synagogue in Colleyville, Texas, including its rabbi, Charlie Cytron-Walker.

The ten-hour standoff ended in a shootout, with all four hostages released and the suspect killed.

Several months ago, the US intelligence community warned of a threat from extremists such as white supremacy supporters who would attempt to commit mass murder.

The agency also warned that Islamic State or its affiliates could make public calls for retaliation because of last week's US special forces raid in Syria that killed the group's leader, Abu Ibrahim al-Hashimi.