US Vice President Joe Biden said on Wednesday he was not running for president, ending months of speculation about his possible White House bid that had reached fever pitch earlier this week.

Speaking at a press conference held outside his previous schedule for Wednesday, Biden said the grieving process over the death of his son Beau Biden "closes the window" on mounting a "realistic" White House bid.

"I've concluded it has closed. I know from previous experience that there's no timetable for this process," Biden said in the White House Rose Garden with President Barack Obama and Biden's wife Jill at his side.

"I believe we're out of time, the time necessary to mount a winning campaign for the (Democratic) nomination," he said, less than 15 weeks before the first electoral event of the nominating process will be held in Iowa.

Biden's decision also ended months-long speculation over whether he would challenge front-runner Hillary Clinton for the Democratic nomination.

Biden's freshest flirtation with a White House run first surfaced on Aug. 1 after a New York Times column claimed that Biden's associates had already started to contact Democratic leaders and donors who had not endorsed Clinton or who had become increasingly concerned about odds of a successful run by Clinton.

The controversy surrounding Clinton's private setup of an email system threatens to obstruct Clinton's otherwise smooth path to the Democratic nomination as an increasing number of voters have begun to see her as untrustworthy.

On the other hand, Biden is currently enjoying unprecedented approval rate since 2009.

However, Biden in previous weeks spoke up on several occasions his concerns of how a presidency bid would impact the family which were still grieving Beau's death and questioned whether the family would have "the emotional energy to run."

Speculation about Biden's White House bid ran amok earlier this week as local media cited sources as saying that Biden's announcement of his candidacy would be imminent.

Also, in a speech on Tuesday, Biden corrected his account about the decision-making process leading up to the 2011 raid that killed Osama bin Laden, saying he did not oppose the raid, a move seen by many as attempting to dispel a possible criticism should he launched a candidacy.