Washington - Democratic presidential hopeful Barack Obama has chosen veteran Delaware Senator Joe Biden as his pick to campaign alongside him as the vice presidential candidate, multiple US media outlets reported early Saturday, citing party sources.

Obama's campaign has yet to confirm the pick, which has been the hot topic of US media all week amid speculation over which of a handful of likely politicians would get the nod.

The official announcement was expected to come within hours. Obama was to notify supporters via email and text message ahead of an afternoon campaign stop in Springfield, Illinois, where the senator got his political start in the state house and launched his presidential bid last year.

The Obama-Biden Democratic ticket was expected to make its first public appearance at the Springfield rally. ABC News reported that a secret service detail had already been dispatched to Biden's house in Delaware.

Biden, 65, is a longtime senator from Delaware and chairman of the upper house's Foreign Relations Committee. He is an old hand at international relations who travelled to Georgia this month in the middle of the country's conflict with Russia, and will likely appeal to voters sceptical of Obama's foreign affairs credentials.

But Biden will come under fire for some comments made about Obama during his own brief presidential run this year. Biden questioned whether the 47-year-old Illinois senator was experienced enough, arguing the presidency was not "on-the-job training."

"There has been no harsher critic of Barack Obama's lack of experience than Joe Biden," a spokesman for Republican rival John McCain said in the campaign's first official reaction Saturday morning.

The Democrats gather at their party convention in Denver, Colorado, beginning Monday to formally nominate Obama. Biden, if he is the vice president, is slated to speak Wednesday night.

Speculation over who would be Obama's running mate had reached a feverish pitch Friday night. Broadcaster CNN even showed live shots of several of the likely candidates' homes in hopes of some sort of clue.

The rumour mill now turns to McCain, who is expected to announce his own running mate on Friday, just days before the Republican Party convention begins on September 1 in St Paul, Minnesota.

Likely candidates for McCain include: Mitt Romney, a former Massachusetts governor who came closest to stealing the party nomination from McCain; Minnesota Governor Tim Pawlenty; and former Pennsylvania governor Tom Ridge.