WASHINGTON — The top American military commander in the Middle East said today that officials will probably need some time this summer to reassess the situation in Iraq before drawing down more troops.

Admiral William Fallon said he expects General David Petraeus will suggest to him that come midyear, “it’s prudent to make an assessment of where we are.”

Admiral Fallon said General Petraeus, who oversees combat in Iraq, has the daunting task of making sure Iraqi security forces are ready to take over. The goal will be to try not to lose the momentum American forces have gained, he said.

“We’ll take actions based on the conditions that we find,” Admiral Fallon told the Senate Armed Services Committee. “And if conditions change, I expect there will be a reconsideration.”

By July, the Pentagon is on track to complete its reduction from 20 to 15 active-duty brigades deployed to Iraq.

General Petraeus, who is slated to testify before Congress on April 8-9, has said he favors a “period of assessment” after that so as not to lose the security gains made in recent months. General Petraeus is expected to ask President Bush to wait until as late as September to decide whether to bring home more troops.

Admiral Fallon cautioned that much of the timelines discussion is speculation.

“I think nothing is written in stone,” he said.

During the hearing, several senior lawmakers said they were deeply disappointed that Baghdad opened its doors this week to President Ahmadinejad of Iran. Washington blames Iran for allowing the flow of weapons and foreign fighters into Iraq through its borders, including powerful roadside bombs that have killed countless American troops.

“I think it’s offensive,” the chairman of the Armed Services Committee, Senator Levin, Democrat of Michigan, said. “They’ve got every right to invite whomever they want. They’re sovereign. But we have a right to express an opinion about it.”

Mr. Ahmadinejad, the first Iranian leader to visit Iraq since Iran’s 1979 Islamic Revolution, said American power is crippling the region.

Admiral Fallon called Mr. Ahmadinejad’s visit a “mixed bag” because it presented an opportunity for Baghdad to push the Islamic leader to stop the flow of weapons and start working with coalition forces.

“From our perspective, we are not going to help resolve the problems inside that country without assistance from outside,” he said.

Admiral Fallon said there has been a dramatic decrease of violence in Iraq, but warned that such gains are not irreversible, adding, “I see this on an upward vector.”

Still, he noted in his prepared remarks that “multiple strains of violent extremism remain a threat to the government and populace and some of these groups benefit from external support,” namely Iran and Syria.

In his testimony, Admiral Fallon blames Iran and Syria for fanning the flames of global terrorism. He said Tehran is providing training, money, and weapons to both Shiite militia groups in Iraq, as well as Hezbollah and Hamas, which the American government regards as terrorist organizations.

Syria, he said, continues to meddle in Lebanon and support Hezbollah, while allowing the flow of suicide bombers and foreign fighters into Iraq through its borders.