By Kanishka Singh

WASHINGTON (Reuters) -An advisory from the U.S. Department of Homeland Security warned on Sunday of a "heightened threat environment in the United States" following overnight U.S. military strikes on Iran's nuclear sites.

Rights advocates have already noted heightened Islamophobia and antisemitism in the U.S. since the start of U.S. ally Israel's war in Gaza following an October 2023 attack by Palestinian militant group Hamas.

The DHS issued Sunday's bulletin through its National Terrorism Advisory System and said the ongoing Iran conflict was causing the heightened threat environment.

Low-level cyber attacks against U.S. networks by "pro-Iranian hacktivists" are likely, the bulletin said, adding cyber actors affiliated with Iran's government may also conduct cyber attacks.

"The likelihood of violent extremists in the Homeland independently mobilizing to violence in response to the conflict would likely increase if Iranian leadership issued a religious ruling calling for retaliatory violence against targets in the Homeland," the bulletin said.

U.S. President Donald Trump said late on Saturday he "obliterated" Iran's main nuclear sites with massive bunker-busting bombs, joining Israel's assault against its Middle East rival. The Israel-Iran war began with Israel's attacks on Iran on June 13.

Local U.S. authorities said late on Saturday they remained vigilant, adding there were no known credible threats. In Los Angeles, Washington and New York City, law enforcement agencies said they stepped up patrols and deployed additional resources to religious, cultural and diplomatic sites.

The Israel-Iran war could contribute to U.S.-based individuals plotting additional attacks, the bulletin said, noting earlier attacks that it called antisemitic and anti-Israeli.

Recent U.S. incidents that have raised alarm over antisemitism and anti-Israel attitudes include the fatal Washington shooting of two Israeli embassy employees and a Colorado attack that wounded eight people when a suspect threw incendiary devices into a pro-Israeli crowd.

Incidents that increased concerns about anti-Muslim prejudice include the fatal stabbing of a 6-year-old Palestinian child in Illinois, the attempted drowning of a 3-year-old Palestinian American girl in Texas and a violent mob attack on pro-Palestinian protesters in California.

(Reporting by Kanishka Singh in Washington; Editing by Lisa Shumaker and Nia Williams)