WASHINGTON -- U.S. President Joe Biden led a moment of silence at the White House on Thursday in honour of the victims of separate shootings that killed 11 people at a Southern California ballroom dance hall and and seven others at two mushroom farms in the northern part of the state.

"Our prayers are with the people of Monterey Park and Half Moon Bay," the president said at a Lunar New Year reception.

Biden said he had spoken with Brandon Tsay, 26, who was at a second dance hall a few miles from the scene of the tragedy in Monterey Park when the same gunman entered, brandishing his weapon. Tsay disarmed the gunman, who then fled.

Biden praised Tsay's courage and called him a "genuine hero."

The shootings Saturday night and Monday came during what usually are joyful Lunar New Year celebrations and sent fear through Asian American communities already dealing with increased violence directed at them, some of it due to misinformation about the coronavirus.

Authorities have said Huu Can Tran opened fire late Saturday on a mostly elderly crowd of dancers at the Star Ballroom Dance Studio in Monterey Park. Nine people also were wounded. Tran, 72, was later found dead from a self-inflicted gunshot wound.

Days later, farmworker Chunli Zhao, 66, opened fire at two mushroom farms in Half Moon Bay, killing seven current and former co-workers, police said.

The White House had scheduled its Lunar New Year celebration before the shootings.

Biden said both communities "will be affected by what they saw for the rest of their lives," adding that, "as a nation, we have to be there for them."

On Sunday, Biden ordered American flags on federal facilities lowered to half-staff through sunset Thursday out of respect for the victims in Monterey Park. He said Thursday that he has been in touch with California Gov. Gavin Newsom. He also sent Vice President Kamala Harris, a native of California, to Monterey Park on Wednesday to offer condolences on behalf of the government.

Biden had been in California on Jan. 19, just two days before the dance studio shooting, to survey flood damage along the state's central coast following days of heavy rains. He spoke with Tsay earlier this week.

"Thank you for taking such incredible action in the face of danger," Biden told Tsay in a brief video of the conversation that the White House shared Thursday on Twitter. "I don't think you understand how much you've done for so many people who are never even going to know you."

Tsay replied that he was still processing what had happened.

"For you to call, that's just so comforting to me," Tsay told the president.