US President Donald Trump met his Russian counterpart Vladimir Putin at a second, previously undisclosed meeting at the G20 summit, it has emerged.

The conversation came hours after their first official meeting at the summit in Germany on 7 July.

The pair's first discussion was originally scheduled to last just half an hour but stretched on for more than two.

White House spokesman Sean Spicer and National Security Council spokesman Michael Anton confirmed that Trump and Putin spoke privately at a dinner for world leaders and their spouses at the summit in Hamburg.

Anton would not specify the duration of the meeting, but said the discussion was casual and should not be characterised as a "meeting" or even a less formal, but official, "pull-aside."

"A conversation over dessert should not be characterized as a meeting," he said.

On Tuesday evening, Trump took to Twitter to defend the meeting.

Trump and Putin's relationship has been under scrutiny since the election campaign, when Trump repeatedly praised the Russian as a strong leader and publicly encouraged him to hack then-rival Hillary Clinton's emails.

Trump aides have since said he was joking.

US intelligence officials have concluded that the Russian government meddled in the 2016 election in order to help Trump.

Trump has repeatedly cast doubt on their findings and dismissed investigations into potential collusion between his campaign and Moscow as a "witch hunt."