Joe Biden 'cannot say' if he will run for US president
US Vice-President Joe Biden says he does not know if he has the "emotional energy" to run for president.
Mr Biden said at a foreign policy lecture in Atlanta: "I can't look you straight in the eye and say now, 'I know I can do it'."
The 72-year-old Democrat lost his son, Beau, to cancer earlier this year.
The media have watched and waited for weeks for a hint that Joe Biden will challenge the Democrat front-runner Hillary Clinton.
The BBC's Laura Bicker in Washington says he has finally given an insight into his deliberations - telling those gathered in Atlanta that, unless he was able to devote his whole heart and soul to the endeavour, it would not be appropriate.
He said: "The most relevant factor in my decision is whether my family and I have the emotional energy to run. The factor is, can I do it?
"The honest to God answer is I just don't know."
Our correspondent says some Democrats have urged Mr Biden to run as Hillary Clinton's campaign appears to be struggling with questions over her use of emails when she was Secretary of State.
Mr Biden failed in his bids for the White House in 1988 and 2008 before becoming Barack Obama's running mate.