Obama announced the new strategy during a trip to the Marine Corps base at Camp Lejeune in the state of North Carolina.
The maintenance of a residual force for a period of time in the country did not come as a surprise, but some in Obama's own party have questioned the size of it.
"When they talk about 50,000, that's a little higher number than I had anticipated," Harry Reid, the senate majority leader, said.
Some Republicans also criticised the announcement, with John Boehner, the House Republican leader, saying that while such proposals may have sounded good during the election campaign "I do think it's important that we listen to those commanders and our diplomats who are there to understand how fragile the situation is."
Officials told AP that national elections in Iraq set tentatively for December could be postponed until 2010, which could be why Obama's timetable for withdrawing
combat troops has also been extended by a few months.
|An existing agreement under Bush said
all forces should leave by 2011 [AFP]
Friday's Iraq announcement came a day after he unveiled an ambitious budget that promises a major overhaul of America's costly healthcare system and sets aside an additional $750bn to help rescue America's troubled financial system.
There are currently about 142,000 US troops stationed in Iraq.
An existing US-Iraq agreement, the Status of Forces Agreement, or Sofa, negotiated under George Bush, the former president, already called for US troops to withdraw from Baghdad and other cities by the end of June this year, with all American forces out of the country by the end of 2011.
More than 4,250 US military members have died since the war began in March 2003, though US military deaths plunged by two-thirds in 2008 from the previous year.
Some analysts have attributed the fall in casualties to improving security after a troop build-up, or so-called surge, in 2007.