Donald Trump jnr met during the 2016 campaign with a private military contractor and an adviser to Middle Eastern leaders, both of whom have since become a focus of investigators working for Special Counsel Robert Mueller, a lawyer for US President Donald Trump's eldest son said.

Erik Prince, an informal adviser to the Trump campaign and former head of Blackwater, and George Nader, a veteran operative who has advised the United Arab Emirates and helped American contractors secure business in the Middle East, met Trump jnr at Trump Tower to discuss a social media proposal, lawyer Alan Futerfas said. "They pitched Mr Trump jnr on a social media platform or marketing strategy. He was not interested and that was the end of it," Futerfas said.

Also at the meeting was Joel Zamel, the CEO of a social media company called WikiStrat. But he neither offered social media services to the Trump campaign, nor was he asked to help the campaign, his lawyer, Marc Mukasey, said.

Mukasey also called it "misguided" to say Zamel conducts "social media manipulation," as suggested by the New York Times, which first reported the meeting. The newspaper said the August 2016 meeting was convened primarily to offer assistance to the Trump campaign and that the outreach suggests countries other than Russia may have offered to help get Trump elected.

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Mueller's investigators have been looking into a later meeting, in January 2017 in the Seychelles, that Nader and Prince held with Abu Dhabi's crown prince, Mohammed bin Zayed Al Nahyan, and Kirill Dmitriev, a Russian banker with close ties to the Kremlin. Nader is a witness in Mueller's ongoing investigation.

AP reported in March that Nader had been convicted on 10 counts of sexually assaulting a minor in the Czech Republic in 2003. Employees for Prince's previous firm, Blackwater, were implicated in the deaths of 14 civilians in Iraq in 2007.

Mueller has dug into various facets of the social media and digital efforts in the 2016 election as part of his expansive federal probe.

- AP