"Essentially, we have approved the extension of roughly 20,000 forces, people who are currently in the theatre," Rumsfeld told a Pentagon press briefing on Thursday.
General Peter Pace, vice chairman of the Joint Chiefs of Staff, said the additional forces should be "sufficient for the threat that they now face."
Despite ordering the extension of duty by at least 90 days, Rumsfeld claimed the US was winning the war in Iraq.
He insisted that the insurgency in the flashpoint town of Falluja has been contained and the situation in southern Iraq largely stabilised.
"If you had said to me a year ago, 'describe the situation we will be in today one year later', … I would not have described it the way it happens to be today"
US defence secretary
But the defence secretary acknowledged he had not anticipated a year ago that US forces in Iraq would be suffering losses of the magnitude suffered in the past two weeks.
More than 80 soldiers, as well as several hundred Iraqis, have been killed this month so far.
"If you had said to me a year ago, 'describe the situation we will be in today one year later', … I would not have described it the way it happens to be today," he admitted.
He said the "terrorists and the leftovers of Saddam Hussein's regime have a strategy."
"Their objective is to break the will of the American people and drive the US and coalition forces out of Iraq to foment civil war among Iraqis."
"Their strategy is failing. Far from leaving, coalition forces have responded forcefully to recent attacks," he said.
Rumsfeld also claimed the uprising by Shia leader Muqtada al-Sadr is failing.
The acerbic Rumsfeld also lashed out at Arabic television channels like Aljazeera and Al-Arabiya, accusing them of exaggerating civilian casualties in the US-led offensive in Falluja.
Disputing reports that more than 600 Iraqi civilians have been killed in the battle for Falluja, Rumsfeld dubbed them as "vicious, inaccurate and inexcusable".
The channels have denied the charges.