San Francisco's mayor has admitted to having an affair with the city's disgraced former homelessness chief, as well as accepting thousands of dollars in unreported gifts from him.

Mayor London Breed admitted on Friday to briefly dating Mohammed Nuru, San Francisco's former Director of Public Works, around 20 years ago, leading to calls for her resignation from the city's top job.

Nuru, 58, resigned from his post earlier this week after he was arrested and charged by the FBI in January for a series of alleged public corruption schemes.

The man known as 'Mr Clean', who was tasked with cleaning up the streets of San Francisco and tackling its homelessness crisis, was charged with one count of wire fraud for allegedly attempting to bribe an airport commissioner to win a bid for a restaurant contract at San Francisco International Airport.

Mayor London Breed (above) admitted on Friday to briefly dating Mohammed Nuru, San Francisco's former Director of Public Works, around 20 years ago, leading to calls for her resignation from the city's top job

Nuru, 58, (above) resigned from his post earlier this week after he was arrested and charged by the FBI in January for a series of alleged public corruption schemes

Mayor Breed, 45, has now become embroiled in the scandal shaking up San Francisco when she posted a tweet on Friday linking to a Medium.com post where she reveals the nature of her relationship with Nuru and admits to accepting 'gifts' from him.

Breed said that their romantic relationship was 'two decades ago' but that the pair 'remained close friends'.

'I write this in the spirit of transparency, because in the wake of a scandal at City Hall, I think San Franciscans are entitled to hear directly from their Mayor. And also, quite frankly, to dispel some unfortunate rumors that have already begun to circulate,' she wrote in the post.

'Mohammed Nuru and I have been close personal friends for more than 20 years. We dated for a brief time, two decades ago, long before I ever ran for office. Nevertheless, he, and his now-adult daughters, have remained close friends for all those years.'

She also admitted to accepting a 'gift' to the value of $5,600 from Nuru last year when he paid to repair her car.

Breed said the disclosure of this payment is not legally required but she did it in order to be transparent.

Mayor Breed, 45, has now become embroiled in the scandal shaking up San Francisco when she posted a tweet on Friday (above) linking to a Medium.com post where she reveals the nature of her relationship with Nuru and admits to accepting 'gifts' from him

Breed accepted Nuru's resignation earlier in the week

'And in the spirit of my directive to city employees to be as transparent as possible, I want to note that in my annual 'Statement of Economic Interests,' due this coming April 1, I will voluntarily disclose a 'gift' of approximately $5,600 from Mohammed Nuru,' she wrote.

'Last year, my personal automobile had broken down and Mohammed, acting as my friend, took it to a private auto mechanic... Later, when the car still wasn't working, he helped secure a rental. The estimated value of those things is about $5,600. It had been my intention to sell the car (which is still not running consistently, even after the repairs) and reimburse Mohammed.

'Now, gifts provided 'by an individual with whom the official has a long term, close personal friendship unrelated to the official's position' are not required to be reported under the Fair Political Practices Commission's rules. But since I have not yet been able to sell the car — and given all that has now happened — I have chosen to make this voluntary disclosure.'

The mayor spoke of her 'shock, sadness, disappointment and anger' on hearing the allegations against her long-time friend.

She denies any knowledge of Nuru's alleged corrupt schemes until just before they were made public.

'To be clear: I never asked Mohammed Nuru to do anything improper, and he never asked me to do anything improper,' she wrote.

Nuru, 58, (right) and Nick Bovis, 56, (left) a well-known local restaurant boss, have been charged with allegedly attempting to bribe an airport commissioner to win a bid for a restaurant contract at San Francisco International Airport

She condemned the alleged corruption, writing that if the allegations are true, Nuru's actions mark 'a betrayal of the public trust that cuts to the core of our mission'.

Breed added: 'I will not apologize for dating someone two decades ago. I will not apologize for remaining close friends with him and his family for 20 more years. But neither will I make excuses for any misdeeds.'

Breed's shock announcement came as it had previously emerged that she had received a donation from the second man involved in the scandal, Nick Bovis, 56, a well-known local restaurant boss in 2018 towards her mayoral campaign.

The latest revelations have led to calls for Breed to stand down from her role as mayor, with Supervisor Gordon Mar saying her acceptance of 'unreported gifts' is 'likely illegal, certainly unethical'.

'Mayor Breed's admission of thousands of dollars in unreported gifts from a subordinate is likely illegal, certainly unethical, and part of a culture of casual corruption that is eroding the faith of the public,' Mar said in a statement.

'Given the seriousness of this admission, the direct connection to the central figure in the FBI's investigation into public corruption, I believe we need to put the people of San Francisco first. I believe Mayor Breed should do the right thing and temporarily step back from her duties until a full, independent investigation can be completed.'

Supervisor Dean Preston said he was 'deeply concerned' with the revelations and called for the mayor to appear before the Board of Supervisors.

'I am deeply concerned with revelations today that Mayor London Breed violated San Francisco law by taking thousands of dollars in gifts from a City Hall subordinate,' he tweeted.

Nuru was tasked with keeping the city of San Francisco clean, including cleaning up trash, removing vandalism, repairing sidewalks, and tackling the city's homelessness crisis, in his role as Director of Public Works.

The scandal emerged last month when he and Bovis were arrested after an FBI investigation allegedly unearthed a 'web of corruption'.

The FBI has been investigating Nuru and Bovis since 2018 through wiretaps and undercover officers.

The main alleged corruption uncovered was attempted bribery of an unnamed SFO commissioner in exchange for a restaurant contract at the airport.

According to officials, Nuru and Bovis said they would give the commissioner $5,000 and free travel in exchange for her vote to approve a contract for Bovis' restaurant Spinnerie.

The complaint says the commissioner dismissed the bribes.

Nuru (above) was first arrested and released on the condition that he would cooperate with the FBI investigation and would not tamper with or disclose the investigation. He was rearrested on January 27 after he broke the conditions

Bovis (above) is a well-known local restaurant boss and a donor to Mayor London Breed's 2018 mayoral campaign

In recorded conversations between Bovis and an FBI undercover officer, the complaint shows that Bovis grew suspicious that the FBI may be investigating the pair.

'Well, in my brain, what if this is some sort of FBI sting or something?' Bovis reportedly said.

Four other allegations of corruption continue to be investigated, authorities said.

It includes one where Nuru is believed to have provided Bovis with insider information to bid on contracts for providing mobile housing and toilets for homeless people across the city.

Nuru, who dubbed himself 'Mr Clean', was first arrested in January and released on the condition that he would cooperate with the FBI investigation and would not tamper with or disclose the investigation.

The public servant was arrested again on January 27 after breaking the conditions by disclosing the probe to others.

He was charged with an additional count of making false statements.

'The complaint describes a web of corruption involving bribery, kickbacks, and side deals by one of San Francisco's highest-ranking city employees,' said U.S. Attorney for the Northern District of California David Anderson at a press conference at the time of their arrests.

Nuru (above) who dubbed himself 'Mr Clean', was placed on leave at the Public Works and resigned from the role on Monday

Breed issued a statement at the time of Nuru's arrest saying the allegations were 'extremely serious' and that the city attorney would be conducting 'a thorough review of any implicated city contracts or other decisions'.

Nuru was placed on leave at the Public Works and resigned from the role on Monday.

Breed announced Monday she had accepted his resignation.

Nuru's arrest is the latest in a string of scandals and allegations of corruption to hit the public servant.

In 2004, before entering the public works office, it came to light that his nonprofit organization the San Francisco League of Urban Gardeners was forcing employees to support the campaigns of then-Mayor Gavin Newsom and former District Attorney Kamala Harris.

The City Attorney's Office also accused the organisation of dodgy accounting practices and improper use of city grants.

Then in 2011, Nuru was accused of sexism and racism against black women in a workplace lawsuit. A settlement was reached before the case went to court.

If convicted over the FBI probe, Nuru and Bovis could face up to 20 years in prison.