US federal investigators raided the New York City apartment of former president Donald Trump's personal lawyer Rudy Giuliani early on April 28 as part of a probe into his dealings in Ukraine.

Investigators with search warrants combed Giuliani's residence and a separate office, seizing electronic devices.

They also seized a phone from a Washington attorney, Victoria Toensing, who has advised Trump and worked with Giuliani on Ukraine-related matters, according to news reports.

Giuliani, himself once a top New York prosecutor and then mayor of the city, was a personal lawyer for Trump when the president was impeached in December 2019 for seeking political help from Ukraine.

Giuliani spent months trying to help Trump find dirt in Ukraine on election rival Joe Biden and Biden's son Hunter, during 2018-2020.

But the investigation behind April 28's raid could be honing in on Giuliani doing paid work for Ukrainian businessmen at the same time, violating US laws against unregistered lobbying for foreign entities.

Two Ukrainian-born men, Lev Parnas and Igor Fruman, who helped Giuliani in those efforts were arrested in October 2019 and charged with illegally funnelling large donations to a pro-Trump fundraising committee.

Giuliani's son Andrew Giuliani condemned the raid outside his father's apartment building, calling it "disgusting" and accusing the Justice Department of a political agenda.

"Anybody, any American, whether you're red or blue, should be disturbed by what happened here today, by the continued politicisation of the Justice Department," he told reporters.

"If this can happen to the former president's lawyer, this can happen to any American. Enough is enough," he said.

Giuliani's own attorney, Robert Costello, told the New York Times that the raids on April 28 were "legal thuggery".

"Why would you do this to anyone, let alone someone who was the associate attorney general, United States attorney, the mayor of New York City and the personal lawyer to the 45th president of the United States?" he asked.

According to reports, the Federal Bureau of Investigation (FBI) and the Department of Justice's prosecutor in New York had sought search warrants for Giuliani's phones last year, but were denied that while Trump remained in office, until January 20.

Under the new attorney general, Merrick Garland, in the administration of now-President Biden, the warrants were evidently approved.

The FBI and Justice Department declined to comment on the raids or the investigation.

Investigations of lawyers, especially those who represented a president, are highly sensitive, given protections they have for privileged dealings with clients.

Nevertheless, a previous personal attorney for Trump, Michael Cohen, was sent to prison in 2019 after pleading guilty to charges including paying hush money to two women who said they slept with Trump, for tax fraud and for lying to Congress.

The impeachment investigation into Trump revealed extensive involvement by Giuliani in Ukrainian affairs. He appeared to take part in a push by powerful Ukrainian business and political figures to have Trump remove the US ambassador, Masha Yovanovitch.

He also helped pressure the government in Kiev to provide information that would support Trump's baseless claim that his Democratic election rival Biden was involved in corrupt dealings in the country.

On January 11, the US Treasury imposed sanctions on four Ukrainians who it said helped Giuliani in the effort to smear Biden ahead of the 2020 presidential election.

The Ukraine affair isn't the only legal case facing Giuliani.

In January he was sued by a voting machine maker for $1.3 billion for spreading claims after the November presidential election that its machines were part of alleged massive voting fraud that caused Trump's loss.

No significant fraud has been demonstrated in the election.