WASHINGTON – President Donald Trump on Wednesday threatened to use his Environmental Protection Agency to warn San Francisco that the city's ongoing struggle with homelessness is violating environmental rules.

“It’s a terrible situation that’s in Los Angeles and in San Francisco,” Trump told reporters flying back to Washington with him on Air Force One. “And we’re going to be giving San Francisco – they’re in total violation – we’re going to be giving them a notice very soon.”

It was not immediately clear which environmental laws Trump was referring to, or on what authority the Environmental Protection Agency would act. Trump, who was traveling in California on Tuesday and Wednesday for campaign fundraisers, said pollution was flowing into the ocean because of waste in storm sewers and he cited used needles.

'This wall can't be climbed':Trump touts design, signs wall during visit to U.S.-Mexico border

Trump has significantly ramped up his rhetoric criticizing homelessness in the state.

“EPA is going to be putting out a notice,” he said. "They’re in serious violation."

He added, “They have to clean it up. We can’t have our cities going to hell.”

Trump took aim at Los Angeles and San Francisco over their problems with homelessness earlier in the trip and said he might create a task force to deal with the crisis.

“We can’t let Los Angeles, San Francisco and numerous other cities destroy themselves by allowing what’s happening," he told reporters on Tuesday.

He has criticized other major cities in recent months as well. Over the summer, Trump came under scrutiny for calling Baltimore a “disgusting, rat and rodent infested mess.”

Contributing: Michael Collins, USA TODAY