U.S. President Donald Trump faced legal challenges from women on two fronts on Tuesday as a defamation lawsuit brought by a former Apprentice TV show contestant moved forward and a former Playboy model who said she had an affair with Trump sued to undo a confidentiality agreement.

The developments increased legal pressure on Trump, who during and after the 2016 presidential campaign was accused of sexual misconduct by more than a dozen women, allegations he denied.

A New York state judge on Tuesday denied a bid by Trump to toss a defamation lawsuit by Summer Zervos, a former contestant on NBC's The Apprentice, raising the prospect that he might have to answer questions about his behaviour in court.

Justice Jennifer Schecter in Manhattan rejected Trump's claim that he was immune from being sued, finding "absolutely no authority" to dismiss litigation related "purely to unofficial conduct" solely because he occupied the White House.

"No one is above the law," the judge wrote in her ruling.

Playboy model Karen McDougal, meanwhile, sued a media company that she said paid her $150,000 US to keep quiet about an affair that she said she had with Trump.

That lawsuit was the second time this month that a woman challenged legal arrangements to prevent discussions about affairs women said they had with Trump. Both involved payments that legal experts have said could equate to in-kind contributions to Trump's campaign in violation of federal election laws.

The White House did not immediately respond to a request for comment.

Trump has denied all accusations of misconduct, and called his accusers "liars."

Zervos met Trump when she became a contestant on "The Apprentice" in 2005. She said Trump kissed her against her will at a 2007 meeting in his New York office, and later groped her in a Beverly Hills hotel at a meeting about a possible job.

Zervos said Trump's denials amounted to defamation and that being branded a "liar" caused diners to stay away from her restaurant in California.

John Diamond, a professor at the University of California Hastings College of the Law, said Trump would have to testify under oath if he has to defend himself.

Claims Enquirer killed story to benefit Trump

McDougal filed her lawsuit in Los Angeles Superior Court against American Media Inc, publisher of the National Enquirer, whose head, David Pecker, has described Trump as a "personal friend."

McDougal said American Media paid her $150,000 in 2016 for the rights to her story that she had an affair with Trump in 2006 and 2007 and then never published it.

McDougal said her lawyer at the time, Keith Davidson, secretly negotiated with Michael Cohen, Trump's personal lawyer, and that Cohen was in consultations with American Media on the agreement.

Cohen has acknowledged arranging to pay to silence another woman, porn performer Stormy Daniels. That payment was also made during the election campaign.

Davidson said on Tuesday that he never had contact with Cohen during the McDougal negotiations.

Karen McDougal is shown at a 2004 event in Hollywood, Calif. McDougal is looking to speak about her interactions with Trump, and claims she has that right since the National Enquirer never published her interview. (Cherie Steinberg/Getty Images) "False allegations have been levied against me and I look forward to responding to them in an appropriate forum," he told Reuters.

McDougal is asking the court to declare the agreement null and void. She said she was tricked into signing it, that it was intended to illegally influence the election and because it violates public policy against using threats of legal action to get someone to stay silent on issues of public concern.

She also called the agreement an illegal donation from AMI to the Trump campaign in violation of federal election law.

American Media said in a statement the company has a valid contract with McDougal and looks forward to reaching an amicable resolution. It said she has been free to respond to press inquiries about her relationship with Trump since 2016 and that the suggestion that American Media silenced her is without merit.

Davidson declined to comment, citing attorney-client privilege. Cohen did not respond to a request for comment.

Daniels, whose real name is Stephanie Clifford, sued the president on March 6, stating Trump never signed an agreement for her to keep quiet about an "intimate" relationship between them. Daniels received $130,000 under that agreement.

Neither Trump nor the White House have commented on the subject of the payment to Daniels.

Trump's election hopes appeared to be threatened in October 2016 after Access Hollywood production footage was released in which he bragged about sexually assaulting women. But rival Hillary Clinton was soon confronted with her own challenges as the FBI publicly announced it was taking a look at new information surrounding her use of a private email server while Secretary of State, an investigation that had been closed months earlier.