Rudy Giuliani, attorney for U.S. President Donald Trump, arrives for the White House Sports and Fitness Day event on the South Lawn of the White House in Washington, May 30, 2018.

Lawyers for President Donald Trump are preparing written responses to questions sent by special counsel Robert Mueller, a person familiar with the matter told NBC News Thursday.

The questions, which have been sent back and forth between the two parties and refined for months, will center around Mueller's investigation of possible collusion between Trump's associates and the Russian government, the source said.

CNN reported the action by Trump's lawyers earlier Thursday afternoon.

The question-and-answer phase between Trump's legal team and Mueller follows months of negotiations between the two sides over the format and subject matter of a potential interview with the president, and could signal a major shift in the pace of the ongoing probe.

Mueller may send more questions back to Trump's lawyers after the first round are answered, CNN reported. Additionally, the possibility of a sit-down interview between Trump and Mueller's investigators is still being debated.

"There are ongoing discussions with the Office of Special Counsel and we cannot discuss publicly the contents of those discussions," Trump's lawyer Jay Sekulow told NBC News on Thursday.

A spokeswoman for another of Trump's lawyers, former New York Mayor Rudy Giuliani, did not immediately respond to a request for comment. A spokesman for the special counsel declined to comment on the report.

Mueller was appointed as special counsel in May 2017 to investigate Russian interference in the 2016 presidential election. The team is also looking into whether Trump obstructed justice, possibly by firing former FBI Director James Comey.

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