( BBC ) - The top US military leader in Iraq, Gen David Petraeus, has recommended a suspension of troop withdrawals after July to protect gains in Iraq.
Gen Petraeus praised "significant" but "uneven" improvements in security and said troop levels would need a period of evaluation over the summer.
He also said the recent Iraqi operation in Basra was "not adequately planned".
He and Ambassador Ryan Crocker gave an update to Congress on the Iraq "surge", on the first of two days of hearings.
They also came face to face with the three senators vying to succeed George W Bush as president this November.
John McCain, the Republicans' choice as candidate, was positive about the situation in Iraq while Barack Obama and Hillary Clinton, the rivals for the Democratic candidacy, pressed for withdrawal.
Gen Petraeus and Mr Crocker began on Tuesday by testifying before the Senate Armed Services Committee, then moved on to the Senate Foreign Relations Committee.
Gen Petraeus said security was better than the situation at the time of his last report to Congress in September and significantly better than before the start of the US troop surge at the beginning of last year.
But while there had been real progress, it was "fragile and is reversible", he said.
The planned "drawdown" of about 20,000 troops should continue to July but afterwards there should be a 45-day "period of consolidation and evaluation", he said.
He could not say how many US troops would be in Iraq at the end of the year. The US currently has 160,000 troops in Iraq.
Mr McCain said he believed there was a genuine chance of success in Iraq and that a withdrawal could result in a failure that might require the US to return later for a far more costly war.
"Our allies, Arab countries, the UN and the Iraqis themselves will not step up to their responsibilities if we recklessly retreat," he said.
When he asked Gen Petraeus about the recent Iraqi-led operation against militias in Basra, the US commander said it had not been "adequately planned or prepared".
Iraq sent thousands of troops to Basra in a failed attempt to force the Shia Mehdi Army militia into submission. Hundreds died in heavy fighting.