Top Democrats in the US Congress have warned President George W Bush that he does not have the authority to go to war with Iran.
Ms Pelosi says it is time for Congress to exert its authority
Washington is in dispute with Iran over its nuclear programme, and senior US officials have accused it of supplying weapons to Shia insurgents in Iraq.
The warning came ahead of a House of Representatives vote likely to condemn the recent surge of US troops in Iraq.
The Senate is due to vote on the troop plan in an unusual Saturday session.
There has been concern recently over the president's rhetoric on Iranian activity in Iraq.
House of Representatives speaker Nancy Pelosi acknowledged Mr Bush has said he wants a diplomatic solution to the rift with Iran, saying: "I take him at his word."
But she also said that Congress should assert itself "and make it very clear that there is no previous authority for the president, any president, to go into Iran".
Meanwhile, the House of Representatives is on Friday expected to vote on a non-binding resolution opposing Mr Bush's decision to send an extra 21,500 US troops to Iraq to try to restore stability.
The vote comes after days of fierce debate on the issue in what has been the first full debate in the House since the Democrats took control of Congress in November.
The resolution states that the House "will continue to support and protect" troops in Iraq but that it "disapproves" of the troop increase.
The Senate is holding its own vote - in an unusual Saturday session - on whether to begin debate on the resolution.
Previous Senate attempts to debate the anti-troop surge resolution have been met with delaying tactics from Republican members.
Mr Bush plans to send 21,500 additional US troops to Iraq
Mr Bush faces the possibility that both chambers of Congress will repudiate his Iraq policy says the BBC's Justin Webb in Washington.
Although the upcoming Congressional votes are non-binding, the president needs the legislators to support his $93bn (£48bn) emergency troop funding measure.
"Our men and women in uniform are counting on their elected leaders to provide them with the support they need to accomplish their mission," he said on Thursday.
"Republicans and Democrats have a responsibility to give our troops the resources they need."
But in the debate, Ms Pelosi said there should be "no more blank cheques".
Republican House Minority Leader John Boehner criticised the Democrats' attempts to derail Mr Bush's plans for Iraq.
"While American troops are fighting radical Islamic terrorists thousands of miles away," he said, "it is unthinkable that the United States Congress would move to discredit their mission, cut off their reinforcements and deny them the resources they need to succeed and return home safely".