US President George W Bush has made it clear he will ignore any attempt by the US Congress to question his sending 20,000 extra troops to Iraq.
Mr Bush said he authorised troops in Iraq to target Iranian agents
Mr Bush - who faces tough opposition in Washington to his new Iraq strategy - said he was the decision-maker and would not rethink his plans.
He spoke after the Senate unanimously backed Lt Gen David Petraeus as the new US commander in Iraq.
Meanwhile House Speaker Nancy Pelosi arrived in Iraq on a surprise visit.
Ms Pelosi, a Democrat and vocal critic of Mr Bush's plan to send more troops, held talks with Iraqi Prime Minister Nouri Maliki.
Mr Maliki assured Ms Pelosi that Iraq intended "to assume security missions currently handled by US-led forces in Iraq", a statement from the prime minister's office said.
Ms Pelosi told journalists the meeting with Mr Maliki gave her "a greater understanding of the others' point of view".
US Defence Secretary Robert Gates meanwhile has suggested Congressional resolutions opposing Mr Bush's troops plan "embolden[s] the enemy".
The Senate is due to debate a number of non-binding resolutions next week, some of them sharply critical of the surge.
In a news conference after Gen Petraeus' confirmation, Mr Bush accused opponents of his strategy of "condemning a plan before it's even had a chance to work".
"I'm the decision-maker," he told senior military advisers during a meeting at the White House, the Associated Press news agency reported.
Mr Bush said that he had authorised US forces in Iraq to target Iranian agents there.
"If somebody's trying to harm our troops, or stop us from achieving our goal, or killing innocent citizens in Iraq, we will stop them," he said.
Gen Petraeus, who has spent two of the last four years in Iraq, replaces Gen George Casey.
A career soldier, he graduated from the West Point military academy in 1974 and was commissioned in the infantry.
He has served in Europe and the Middle East and was commander of the 101st Airborne Division during the invasion of Iraq.
Speaking to the Senate armed forces committee on Tuesday, Gen Petraeus backed Mr Bush's plan to deploy additional troops, but warned that the task in Iraq was a tough one.
"The situation in Iraq is dire. The stakes are high. There are no easy choices. The way ahead will be very hard," he said.
"But hard is not hopeless."
Mr Bush wants to commit more than 21,500 additional troops, mainly to violence-hit Baghdad, in an effort to improve security and end sectarian clashes.
But his plan continues to face considerable opposition both from Democrats and members of his own Republican party, who want an earlier transfer of security to Iraqi leaders.