FILE PHOTO: Special Counsel Robert Mueller departs after briefing the House Intelligence Committee on Capitol Hill in Washington, U.S., June 20, 2017. REUTERS/Aaron P. Bernstein

WASHINGTON (Reuters) - Special Counsel Robert Mueller has issued grand jury subpoenas seeking testimony from public relations executives who worked on an international campaign organized by former Trump campaign adviser Paul Manafort, NBC News reported on Friday, citing people familiar with the matter.

This is the first indication that Mueller, who is investigating possible collusion between President Donald Trump’s campaign team and Russia, is beginning to compel witness testimony before the grand jury, NBC said.

Russia’s government has denied interfering in the election and the president has denied collusion took place.

One executive whose firm received a subpoena told NBC Mueller’s team is looking at the lobbying campaign, which ran from 2012 to 2014. Some of the firms involved in the campaign received subpoenas for documents weeks ago, and Mueller’s team is now seeking testimony, NBC quoted him as saying.

Manafort, whose Virginia apartment was raided by FBI agents last month, is a key figure in the Mueller probe. The longtime political consultant and lobbyist is being investigated for possible money laundering and has been targeted as someone who might testify against former colleagues, two people familiar with Mueller’s work have said.