Barack Obama, the US president, has formally announced his 2012 re-election bid, saying the country needed "to protect the progress" it had made.

"Today, we are filing papers to launch our 2012 campaign," Obama said in a statement on Monday.

He explained that as his administration and supporters across the country "fight to protect the progress we've made - and make more - we also need to begin mobilising for 2012, long before the time comes for me to begin campaigning in earnest".

Obama said that in coming days, his supporters will begin forging a new organisation in cities and towns across the country.

"And I'll need you to help shape our plan as we create a campaign that's farther reaching, more focused, and more innovative than anything we've built before," he said.

The announcement was preceded by a video, which featured a group of presidential supporters talking about the importance of continuing the job the president started following his election in 2008.

"This will be my final campaign, at least as a candidate," Obama said in the statement.

"But the cause of making a lasting difference for our families, our communities, and our country has never been about one person. And it will succeed only if we work together."

Political observers expect the Obama campaign to raise an unprecedented $1bn for the race.

Several Republicans are considering running against Obama but have not made their candidacies official. Early polls are showing Obama leading potential Republican rivals.

Obama was expected to file campaign papers with the Federal Election Commission as early as Monday, which would allow him to start raising campaign funds.

While running for president as a senator from Illinois, Obama raised a record $750m to win the 2008 election.