But he also kept up with his customary bluster on Wednesday, insisting that only "relatively small numbers of people" were causing the violence.
"There's nothing like an army" or "large elements of hundreds of people trying to overthrow or to change the situation," Rumsfeld maintained at a Pentagon press briefing.
Answering persistent queries on the worsening situation in Iraq, Rumsfeld, however, conceded the situation was serious.
"You have a mixture of a small number of terrorists, a small number of militias, coupled with some demonstrations and some lawlessness. It's a serious problem, and the problem's being worked on," he said.
In a tacit admission of the growing concern in Washington, Rumsfeld said the Pentagon may delay the scheduled imminent return home of some US troops from Iraq in order to deal with the crisis.
The Pentagon had earlier promised troops that they would be brought back home after a year's stint in Iraq.
The US is currently near the end of a massive rotation of weary troops home from Iraq and fresh ones into the country. There are currently about 135,000 US troops in Iraq.
"Because we are in the midst of a major troops rotation, we have planned increase in the number of US troops in the Central Command area of responsibility and indeed in Iraq," Rumsfeld said.
"We are taking advantage of that increase, and we will likely be managing the pace of the redeployments to allow those seasoned troops with experience and relationships with the local populations to see the current situation through," he said.