(Reuters) - Special Counsel Robert Mueller wants President Donald Trump to commit to a follow-up interview to written answers to questions in his probe of any coordination between Trump campaign members and Russia in the 2016 U.S. election, Rudy Giuliani, who is representing the president, said on Thursday.

Giuliani, who said talks between the two sides were continuing, saw Mueller’s stance as a hardening in the position prosecutors are taking after offering to allow Trump to answer questions in writing.

“I thought we were close to having an agreement until they came back with, ‘You have to agree now that you’ll allow a follow-up,’ and I don’t see how we can do it,” Giuliani told Reuters.

Peter Carr, a spokesman for Mueller, declined to comment.

Lawyers for Trump have been negotiating over a potential interview with Mueller’s team since last year in the U.S. investigation of Russian meddling in the presidential election, which Moscow denies. Trump has denied any campaign collusion, calling the Mueller probe a “witch hunt.”

In a letter to Trump’s lawyers last week, Mueller expressed a willingness to accept written responses on questions about collusion, but did not rule out a possible interview as a follow-up, a person familiar with the matter told Reuters on Tuesday.

After receiving the written responses, Mueller’s investigators would decide on a next step, which could include an interview with Trump, the person said.

But Giuliani said on Thursday that Mueller’s team had stiffened its position in the latest talks.

“They want a commitment” to a follow-up interview, Giuliani said. “We’ve said no, and let’s see how they deal with it.”

Giuliani has described a possible interview with Mueller as a potential “perjury trap,” an opportunity to catch Trump making a false statement under oath, and legal experts have also suggested that Trump could open himself up to trouble.

Mueller is also investigating whether Trump may have tried to obstruct the Russia investigation after winning office, but Giuliani said on Thursday that no questions on the obstruction issue would be part of the first round of questions.

If negotiations break down with Mueller, a subpoena could be issued for Trump to testify before a grand jury, which Giuliani has said they would fight all the way to the U.S. Supreme Court.

U.S. intelligence agencies concluded that Russia interfered in the campaign, seeking to tilt it in Trump’s favor against Democratic candidate Hillary Clinton by hacking Democratic computer networks and spreading disinformation on social media.