WASHINGTON - US President Barack Obama reportedly gave Vice-President Joe Biden his blessing to run for president in 2016, it was reported.

The two discussed the issue of presidential candidacy during their lunch at the White House on Monday, CNN quoted a senior Democrat as saying.

Obama made clear that he would not stand in Biden's way or counsel him against a run, the senior Democrat was quoted as saying.

Obama had earlier been careful not to pick among the Democratic candidates. But several close aides have jumped from the White House to former secretary of state Hillary Clinton's campaign in recent months, AFP reported.

Her struggles to explain why she used a private email server to conduct state business has raised questions about her status as presumptive nominee.

Democrats have long been concerned about the prospect of a half-hearted primary race leading to Clinton's nomination.

Biden would provide Clinton with stiff competition, but his path back to the White House is not entirely clear, according to AFP.

He has been a presidential candidate twice and twice lost badly.

White House spokesman Josh Earnest did not rule out the possibility that Obama would ultimately chose between his vice-president and his former secretary of state.

"I wouldn't rule out an endorsement," he was quoted as saying.

Speculation about Biden's plans were fueled by reports over the weekend that he had met privately with Senator Elizabeth Warren, an influential voice in the Democratic Party's left wing.