The military objectives of the US troop surge in Iraq "are largely being met", the top US military commander in Iraq, Gen David Petraeus, has said.
He told a Congressional panel that although improvements were "uneven", violence had declined significantly since the surge began in February.
Both Gen Petraeus and US ambassador to Iraq Ryan Crocker have been testifying.
The hearings have been billed as "make or break" for President George W Bush's policy in Iraq.
In his testimony before the joint hearing by the House Armed Services and Foreign Affairs committees, Gen Petraeus said:
- "security incidents", including sectarian violence, had declined since the start of the surge
- he envisioned the withdrawal of some 30,000 US troops by the middle of 2008, beginning with 2,000 marines in September
- he expected a decision on further troop cuts next March
- the situation in Iraq remained "difficult".
Speaking after Gen Petraeus, Mr Crocker said he believed it was possible for the US to see its goals achieved in Iraq.
Meanwhile, at least eight American soldiers died in Iraq, including seven killed in a vehicle accident in western Baghdad.
The US military also said it had killed 12 al-Qaeda fighters in a raid near Samarra, north of Baghdad, and detained three suspected militants.
A record 168,000 US troops are now in Iraq after 30,000 arrived in the surge between February and June.
Gen Petraeus told the committees that troop numbers could be eventually reduced to pre-surge level without jeopardising the security situation.
But he warned that a premature reduction would have "devastating consequences".
"It is possible to achieve our objectives in Iraq over time, although doing so will be neither quick, nor easy," he said.
Gen Petraeus said his testimony had not been cleared by the Pentagon or the White House before he gave it, and that he had written it himself.
Before he began his testimony, House Foreign Affairs Committee Chairman Tom Lantos, a Democrat, said President Bush's policies in Iraq had "created a fiasco".
He said the objective of the surge was to buy time for Iraqi Prime Minister Nouri Maliki to end sectarian violence and secure a political settlement between sectarian groups.
"As best we can see, that time has been utterly squandered," he said.
"We need to get out of Iraq, for that country's sake and for our own."
The Congressional hearings come as a new opinion poll suggested about 70% of Iraqis believe security has deteriorated in the area covered by the surge.
The survey - by the BBC, ABC News and NHK - of more than 2,000 people across Iraq also suggests that nearly 60% see attacks on US-led troops as justified.
However, Mr Maliki said violence since the surge began had fallen 75% in the provinces of Baghdad and Anbar and that a sectarian war had been prevented.
BBC defence correspondent Rob Watson says Gen Petraeus has become something of a talisman for President Bush - the commander he has entrusted to rescue his policy in Iraq.
Last Thursday Democrats seized on a report from a key US security commission recommending a cut in troop numbers by early 2008.
Mr Bush, who decides on force levels in Iraq, has said he will follow advice from commanders on the ground, rather than bowing to political pressure.