The US defence secretary has said there is no timetable for Iraq to prove it has fulfilled commitments required by the US in President Bush's new plan.
Defence Secretary Robert Gates is facing some stiff questions
But Robert Gates said the US would have a "good idea" of the Iraqi government's success before many new troops went in.
The plan, by which more than 20,000 more troops will be sent to Iraq, may begin in earnest around the first week in February, he told a Senate hearing.
Both Democrats and some Republicans have criticised the plan.
Mr Bush plans to send at least 20,000 more troops, saying it will help bring security to Baghdad's streets.
Mr Gates is facing the Senate Armed Services Committee together with the chairman of the Joint Chiefs of Staff, Gen Peter Pace.
But he appears to have had a less hostile reception in the Senate than Secretary of State Condoleezza Rice, who was grilled by the Foreign Affairs Committee on Thursday.
Florida Senator Bill Nelson put this down to the fact that Mr Gates was a "breath of fresh air", having only joined the Bush administration in December.
In his opening statements, Mr Gates acknowledged widespread concern over the strategy.
But he emphasised that US military commanders on the ground in Iraq backed the president's plan.
"They believe it is a sound plan that can work - if the Iraqi government follows through on its commitments," he said.
Responding to questions, he said the new troops would be sent in gradually.
One brigade will go in the middle of this month. A second brigade won't go until the middle of next month," he said.
"And then they will flow at roughly monthly intervals, so that after we have sent in just two or three of the brigades, I think we will... have a pretty good idea whether at least on the military side the Iraqis have stepped up to the plate in terms of fulfilling their commitments.
But Mr Gates did not say what the US would do if the Iraqi government failed to meet the commitments.
He also said there should be no announcement of a troop pullout, whenever this was to happen.
"I think that any time you announce a specific deadline or specific time-line for departing in a situation as volatile as this, you basically give your adversaries the confidence that all they have to do is wait you out," he said.
But both Democrats and Republicans have continued to criticise the strategy.
Democrat Senator Carl Levin, the committee chairman, said he opposed sending in more troops "without condition, without limitation".
"For America to supply more troops while the Iraqi leaders simply supply more promises is not a recipe for success in Iraq," he said.
The hearing also sought to address concerns about possible military action against Iran.
Gen Pace denied that President Bush's promise to tackle Tehran's interference in Iraq would lead to US military operations inside Iran.
"From a military standpoint, no need to cross the Iranian border," he said.
Mr Bush made his announcement of a "surge" force of troops in a televised speech on Wednesday.
Alongside criticism from Democrats - who now control both houses of Congress - several Republicans have publicly expressed their disappointment with the move, which runs counter to recommendations by the bipartisan Iraq Study Group.
The Democrats have promised a non-binding vote in both houses of Congress on the strategy.