WASHINGTON, April 17 -- White HousePress Secretary Sean Spicer said on Monday that President Trump's 2016 tax returns are under audit and will not be released.

"The president is under audit, it's a routine one, it continues, and I think the American public knows clearly where he stands, this was something he made very clear during the election cycle," Spicer said at the White House daily press briefing.

"We are under the same audit that existed, so nothing has changed." said Spicer.

One day earlier, Trump questioned if protesters over his tax returns were being paid, responding to the weekend's nationwide demonstrations seeking for the release of his tax returns.

"Someone should look into who paid for the small organized rallies yesterday," Trump tweeted Sunday. "The election is over!"

"I did what was an almost an impossible thing to do for a Republican-easily won the the Electoral College! Now Tax Returns are brought up again?" he tweeted earlier Monday.

Trump's latest remarks imply that he has no intention to issue his tax returns, some U.S. media reports commented.

Demonstrators in dozens of U.S. cities and towns, including Washington D.C., New York, Philadelphia, Chicago and Palm Beach, where Trump was spending Easter Weekend at his resort Mar-a-Lago, marched on Saturday to demand Trump release his tax returns. Some rallies were joined by thousands of people.

During his campaign and after the election victory, the Trump camp repeatedly refused to release Trump's tax returns, saying Trump's tax returns were under audit. However, many tax experts say Trump is not barred from releasing the information during the audit.

While U.S. presidents are not required to release their tax returns, nearly all U.S. presidents had voluntarily released them since 1970s.

Shortly after Trump's inauguration in January, Kellyanne Conway, senior counselor to Trump, told U.S. media that Trump would not release his tax returns, citing voters' indifference to the issue as one of the reasons.

However, multiple polls have found that the majority of Americans want Trump to release his tax returns. A petition demanding Trump release his tax returns garnered more than 1 million signatures. Many lawmakers, including some Republicans, have also called on Trump to make them public.

April 18 is the traditional tax deadline in the United States.