Donald Trump has vowed to issue the city of San Francisco with an environmental violation, claiming the city’s sewer system is polluting the ocean with used needles and other waste.

The decision marks an escalation in the president’s attacks on major Democrat-held US cities over what he describes as their problems with homelessness, crime and drug use.

Mr Trump’s accusation that homelessness in San Francisco is polluting the surrounding environment comes two days after he controversially announced the repeal of Obama-era clean water regulations limiting the use of polluting chemicals near lakes, streams and wetlands.

“They have to clean it up,” Mr Trump told a reporter aboard Air Force One following a state visit to California. “We can’t have our cities going to hell.

“It’s a terrible situation – that’s in Los Angeles and in San Francisco. We’re going to be giving San Francisco … [the Environmental Protection Agency] is going to be putting out a notice. They’re in serious violation.”

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San Francisco’s mayor London Reed disputed what she described as Mr Trump’s “ridiculous assertions”, calling the city’s combined sewer system “one of the best and most effective in the country.

“No debris flows out into the bay or the ocean,” she said, vowing that all waste in storm drains is filtered out in treatment plants.

This was backed up by the city’s public utilities commission, who also assured the San Francisco Chronicle: “We haven’t had any (recent) problems with syringes.”

It is unclear whether Mr Trump referred to any statistics or factual evidence while telling reporters that used needles were polluting the ocean.

The type of environmental violation threatened by Mr Trump is usually reserved for companies rather than cities and is a civil, non-judicial course of action. The EPA's website states that such a notice does not mean the agency conclusively believes a violation has taken place.

“In San Francisco we are focused on advancing solutions to meet the challenges on our streets, not throwing off ridiculous assertions as we board an airplane to leave the state,” Ms Reed said.

An official count in January found more than 8,000 people are homeless in San Francisco, a suggested rise of 17 per cent since 2017, with job losses reported as the main factor in people losing their homes.

Public injecting of heroin has also been rising, according to the city’s police. The public health department retrieved 164,264 needles in August 2018 alone via a longstanding disposal programme and street cleanups, the New York Times reported.

Ms Reed said the city was providing 1,000 new shelter beds, is working to create a $600m affordable housing bond, and is increasing mental health and drug services. California’s governor Gavin Newsom’s June budget announced spending of around $2.75bn in efforts to address homelessness in the state.

Mr Trump, who visited the city for the first time in his tenure this week, has frequently criticised San Francisco and its congresswoman Nancy Pelosi over the rise in homelessness.

Donald Trump tries to swat fly and uses it as a segue to lament Washington

“Speaking of failing badly, has anyone seen what is happening to Nancy Pelosi’s district in San Francisco,” he wrote on Twitter in July. ”It is not even recognisable lately. Something must be done before it is too late. The Dems should stop wasting time on the witch hunt hoax and start focusing on our country.”

Opponents allege that Mr Trump’s criticisms are part of a concerted effort to paint Democrat-governed cities, such as Los Angeles, San Francisco, Baltimore, Chicago and New York, as crime-ridden ahead of the 2020 election.

In response to Mr Trump’s Wednesday allegations, San Francisco senator Scott Wiener branded him “a slumlord”, and accused him of pushing people into homelessness by stripping health care, food assistance and affordable housing funds.

He called the accusations part of Mr Trump’s “let’s slander the cities campaign”, the San Francisco Chronicle reported.

“I wish Trump would have his EPA enforce against actual environmental problems, but I guess that would be too much to ask,” Mr Wiener wrote on Twitter.

On 12 September, Mr Trump announced he would repeal the clean water act, taking power away from the EPA in the 22 states in which the law was in effect.

The bill was created to limit pollution in a majority of US waterways, protecting about a third of the country's drinking water. With its repeal, farmers will no longer need a permit to use chemical pesticides and fertilisers in areas that could contaminate such waterways, the New York Times reported.