The four-star general called the war there "the most complex and challenging I have ever seen."
He said there have been some improvements in the two months since President Bush's troop buildup began, but "there is vastly more work to be done across the board. ... We are just getting started with the new effort."
Petreus spoke as the Senate debated House-passed legislation to begin withdrawing U.S. troops in October.
Asked about the impact on the effort in Iraq if that legislation passed, Petraeus said, "I have tried to stay clear of the political minefields of various legislative proposals." Bush has said he will veto the bill.
"This effort may get harder before it gets easier," Petraeus told a briefing for reporters at the Pentagon.
The general also said, however, that improvements can be seen both in the capital of Baghdad and the volatile Anbar Province in Western Iraq. Still, he said, these achievements "have not come without sacrifice."
He said that the increasing use of car bombs and suicide attacks, plus the greater concentration of U.S. troops among the population, has "led to greater U.S. losses" as well as increased Iraqi military casualties.
Petraeus sidestepped a direct question on how long U.S. troops would have to remain in Iraq.
"I wouldn't try to truly anticipate what level might be some years down the road," he said.
Still, Petraeus noted, it was "an endeavor that clearly is going to require an enormous commitment over time."
Petraeus said that the situation was made worse by "exceedingly unhelpful activities by Iran and Syria, especially those by Iran."
Asked whether higher-ups in the Iranian government were sanctioning sending weapons and technology to insurgents in Iraq, the U.S. general said it was hard to say. "We do not have a direct link of Iranian involvement," in attacks, he said.
Petraeus also said that while the fledgling Iraqi government is often billed as a unity government among Shiites, Sunnis and Kurds, it actually is not.
"It is not a government of national unity. Rather, it is one comprised of political leaders from different parties that often default to narrow agendas and a zero-sum approach to legislation," the general said.
He said that was one reason why progress has been so slow on deciding how to divide up oil revenues and pass budget and emergency powers laws.
Despite the disappointing pace, Petraeus said he believes that Iraqi Prime Minister Nouri al-Maliki and other leaders "are committed to achieving more in this area in the months ahead."
Petraeus cited slowly improving conditions in Anbar province, noting it had been "assessed as lost six months ago."
He said the increased U.S. presence has enabled Iraqis "to stitch together the fabric of society that was so torn."
But he said improvements, such as the reopening of shops and the return of some residents to their homes, are "often eclipsed by sensational attacks that overshadow our daily accomplishments."
"Iraq is in fact the central front of al Qaeda's global campaign," he said. "Al Qaeda-Iraq remains a formidable foe with considerable resilience and a capability to produce horrific attacks."
"This group's activities must be significantly disrupted at the least for the new Iraq to succeed," he added. "The key to success is disrupting their attacks."