Gates added that it was no surprise that fighters had launched increased attacks in recent days.
"I think that we expected that there would be in the short term an increase in violence as the surge began to make itself felt," he said.
He added that there were other "very preliminary positive signs" that the Baghdad security plan was working but gave no details.
But the request for extra troops comes amid fierce opposition in the Democratic-controlled congress to the increase in troops, and polls that show little public support for expanding the size of the force.
Gates last month assured congress that only about 2,000 to 3,000 support troops would be needed in addition to the combat forces that will add to the 141,000 US troops now in Iraq.
But Gordon England, the deputy defence secretary, told legislators on Tuesday that between 4,000 and 7,000 more support troops were needed for the Baghdad security operation, increasing the $5.6 bn cost by as much as $2 bn.
Gates said there were other requests had been made for additional troops that had not yet been approved.