US Secretary of State Condoleezza Rice has said that Iraqi PM Nouri Maliki is living "on borrowed time", but that she is confident he can give Iraq security.
Ms Rice was pressed by senators about her confidence in Mr Maliki
Ms Rice was testifying to a Senate hearing about President Bush's new Iraq strategy, announced on Wednesday.
Democrats and some Republicans in Congress have criticised the plan, with one senior Democrat, Senator Joe Biden, calling it a "tragic mistake".
As part of the plan, Mr Bush will boost US troop numbers by more than 20,000.
Later Defence Secretary Robert Gates promised to increase the overall strength of the armed services by recruiting an extra 92,000 troops.
Asked about how long troops would stay in Iraq, Mr Gates said he thought it would be months not years.
"I don't think anybody has a definite idea of how long a surge would last," he told a news conference.
"I think for most of us in our minds we're thinking of it as a matter of months, not 18 months or two years."
He added that the US could revise its plan if Iraqi leaders failed to keep to its commitments.
"The timetable for the introduction of additional US forces will provide ample opportunity early on, and before many of the additional US troops actually arrive in Iraq, to evaluate the progress of this endeavour and whether the Iraqis are fulfilling their commitments to us," he said.
Earlier, Ms Rice warned that the US would take action against countries destabilising Iraq.
Her statement came hours after US forces stormed a building in the northern Iraqi town of Irbil.
Iranian and Kurdish officials said the target had been Iran's consulate, and several Iranians had been arrested. Tehran has condemned the raid.
However, sources at the US Pentagon said the building which was raided did not have any diplomatic status.
In a TV address on Wednesday, Mr Bush said fresh troops in Iraq would help to secure Baghdad's streets as part of the new strategy.
He said his country's commitment to Iraq was "not open-ended", and that he expected the government in Baghdad to fulfil its own promises.
While questioning Ms Rice on the plan, Senator Biden, who chairs the Foreign Relations Committee, asked whether she thought Mr Maliki would keep his side of the bargain.
"I have met Prime Minister Maliki. I saw his resolve," she said.
"I think he knows that his government is, in a sense, on borrowed time, not just in terms of the American people but in terms of the Iraqi people."
Ms Rice's testimony was briefly interrupted by an anti-war protester, who shouted: "All lies... More lies... Still lies... Stop the lies... Stop the war!"
Senator Biden said he could not accept the plan.
"Secretary Rice, to be very blunt, I cannot in good conscience support the president's approach," he said, quoted by the Associated Press news agency.
Senator Chuck Hagel, a Republican on the committee, also said the plan was a mistake.
"I think this speech given last night by this president represents the most dangerous foreign policy blunder in this country since Vietnam if it's carried out," he said.
Mr Bush said in his address that the situation in Iraq was unacceptable, and that responsibility for mistakes rested with him.
But the new troops, most of whom would be sent to Baghdad, would help secure neighbourhoods in the capital from insurgents, he said.
He said the effort would succeed where previous operations had failed, because this time troop levels would be sufficient to hold areas that had been cleared.
The Democrats have promised a non-binding vote in both houses of Congress on the strategy.