The problems that plague the Middle East, including Iran's nuclear ambitions and Syria's civil war, require "political, not military" solutions, US Defense Secretary Chuck Hagel said Thursday.

Saying the "old order" was vanishing in the region, Hagel stressed in a speech that the United States would work to promote democratic reform while bearing in mind the "limitations" of American power.

Although the Pentagon chief made clear that Washington had not ruled out potential military action against Iran or Syria, his remarks highlighted President Barack Obama's cautious stance on resorting to armed force in the volatile region.

He said that regional challenges including "the nuclear challenge posed by Iran, dangerous instability in Syria, or the continuing threat of al-Qaeda and other terrorist groups" must be addressed through "coalitions of common interests," including Israel and other allies in the region.

"A common thread woven into the Middle East fabric is that the most enduring and effective solutions to the challenges facing the region are political, not military," Hagel told an audience at the Washington Institute for Near East Policy.

"America's role in the Middle East is to continue to help influence and shape the course of events -- using diplomatic, economic, humanitarian, intelligence, and security tools in coordination with our allies," he said.

Arab uprisings had shaken the established political landscape in the Middle East, he said.

"The old order in the Middle East is disappearing, and what will replace it remains unknown.

"There will continue to be instability in the region as this process plays out and we all must adjust accordingly."

Prospects for stability in the longer run would hinge on the outcome of political transitions in Egypt, Libya and Syria, said Hagel, who traveled to the region last month.

"The best hope for long-term stability relies on countries like Egypt, Libya, and Syria making transitions to democratic rule," he said.

The United States would "remain engaged in helping shape the new order, but we must engage wisely," he said.

"This will require a clear understanding of our national interests, our limitations, and an appreciation for the complexities of this unpredictable, contradictory, yet hopeful region of the world," he said.

The war in Syria was turning "sectarian" and the possibility the state would break apart was "increasing," he said.

The war was putting Syria's "stockpiles of chemical weapons and advanced conventional weapons at risk, and the escalation of violence threatens to spill across its borders," he said.

But Hagel struck a restrained tone on Syria and did not reiterate Obama's declared "red line" warning Damascus not to use its chemical weapons.

The Obama administration has faced renewed calls for intervention after US intelligence agencies said the Syrian regime probably used chemical weapons on a small-scale. But the White House says the spy services are still investigating the allegations.

After the speech, when asked about Syria, Hagel said the administration would not take any action until it had all the facts and alluded to the intelligence disaster in the run-up to the 2003 Iraq war.

"It's fair to say that we're all probably a little wiser today than we were before and when we take action, there is always the reality -- that you accept -- that there may be consequences and unintended consequences may come from that," he said.

"There are also consequences and unintended consequences that come from inaction," he added.

In answering the question on Syria, Hagel joked about his outspoken style before he took over at the Pentagon in February.

He said now he had to watch his words more carefully as he was no longer a senator and he couldn't "speak as irresponsibly as I would like."