Donald Trump has attacked Hillary Clinton’s campaign team for backing an election recount effort based on suspicions foreign hackers rigged the vote in key states, even as the US president-elect made the unsupported claim that “millions” of Americans cast ballots illegally.

Following a weekend decision by the Clinton campaign to participate in a recount in Wisconsin, one of three swing states targeted for new counts by the Green Party’s Jill Stein, Mr Trump took to Twitter on Sunday to attack his former rival.

"So much time and money will be spent — same result! Sad,” Mr Trump wrote. Hours later he added: “In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally."

The president-elect’s claims echoed those he made during the campaign that he was facing a “rigged” system and that millions of Democratic supporters would be voting illegally. Despite the recount effort and Mr Trump’s allegations, however, there has to date been no credible proof of any significant illegal voting benefiting either side. Mr Trump elaborated on his unsubstantiated claims on Sunday evening, tweeting that the media had ignored “serious voter fraud in Virginia, New Hampshire and California” — all states Mrs Clinton won.

The interjections, which came at the end of a long holiday weekend almost two weeks after his election victory, highlighted the Republican’s continuing efforts to assert a mandate that has been undermined by Mrs Clinton’s growing margin in the national popular vote.

With votes still being tallied in California, Mrs Clinton’s lead in the popular vote count hit 2.2m at the weekend out of almost 135m cast. Under the US’s electoral college system Mr Trump won the election thanks to collecting just over 100,000 more votes than Mrs Clinton in three crucial swing states — Michigan, Pennsylvania and Wisconsin.

In Wisconsin, where officials on Friday agreed to conduct a recount, Mr Trump’s margin of victory stands at just over 22,000 votes of nearly 3m cast, according to the Wisconsin Elections Commission. Ms Stein won more than 30,000 votes in the state.

Mr Trump’s campaign manager, Kellyanne Conway, on Sunday accused Democrats of undermining the peaceful transition of power by backing the recount effort.

“Their president, Barack Obama, is going to be in office for eight more weeks. And they have to decide whether they’re going to interfere with him finishing his business, interfere with this peaceful transition … or if they’re going to be a bunch of crybabies and sore losers about an election that they can't turn around,” she told ABC’s This Week.

The salvos came as the president-elect faced fresh questions about how his property empire’s international ties may influence his conduct of foreign policy and more scrutiny of the messy nature of his transition, which has seen his party divided over key appointments.

Ms Conway on Sunday stepped up her attacks on Mitt Romney, who is believed to be in the running for secretary of state, for his repeated criticism of Mr Trump during the campaign.

“The volume [and] intensity of grass-roots resistance to Romney is breathtaking,” she tweeted on Sunday. The 2012 Republican presidential candidate’s main rival for the job of chief diplomat is believed to be Rudy Giuliani, the outspoken former mayor of New York.

While any recount is unlikely to change the election result, Ms Stein and her backers have said they want to address suspicions that foreign hackers could have manipulated the vote by filing illicit absentee ballots electronically.

Leading cybersecurity experts have backed her petition in Wisconsin, calling the possibility of foreign hacking “plausible”. They cite evidence of other alleged interference in the campaign by Russian hackers who US intelligence agencies believe were behind leaked Clinton campaign emails.

People in both major US parties continued to express concerns about interference, from Russia in particular. Marco Rubio, the senator and former Republican presidential candidate, said on Sunday that he would back a congressional inquiry into possible foreign meddling in the election.

“If a foreign government has been involved in injecting chaos into our democratic process, the American people deserve to know that,” he told NBC’s Meet the Press.

“I've never said it’s the Russian government, although I believe it was the work of a foreign government,” Mr Rubio said. But “if you look at what happened during our election … they are very similar to the sort of active measures that you’ve seen the Russians use in the past in places like eastern Europe, to interfere with the elections of other countries. And what we mean by ‘interfere’ is they try to undermine the credibility of the election.”