US special counsel Robert Mueller is leading the probe into alleged ties between Russia and Trump. Pic/AFP
US special counsel Robert Mueller is leading the probe into alleged ties between Russia and Trump. Pic/AFP

A grand jury has issued subpoenas in connection with a June 2016 meeting that included President Donald Trump's son, his son-in-law and a Russian lawyer, two sources said on Thursday, signalling an investigation is gathering pace into suspected Russian meddling in the 2016 US election.

The sources added that US Special Counsel Robert Mueller had convened the grand jury investigation in Washington to help examine allegations of Russian interference in the vote. One of the sources said it was assembled in recent weeks.

Moscow denies any meddling and Trump denies any collusion by his campaign, while regularly denouncing the investigations as political witch hunts. Mueller's use of a grand jury could give him expansive tools to pursue evidence, including issuing subpoenas and compelling witnesses to testify. The Wall Street Journal earlier reported a grand jury was impaneled. A spokesman for Mueller declined comment. A grand jury is a group of ordinary citizens who, working behind closed doors, considers evidence of potential criminal wrongdoing that a prosecutor is investigating and decides whether charges should be brought.