Retired Army Gen. David Petraeus apologized to Congress on Tuesday for sharing classified information with his biographer and mistress, Paula Broadwell. It was his first public testimony before lawmakers since resigning as CIA director.

Petraeus appeared before the Senate Armed Services Committee, offering his recommendations for how the U.S. should address what he called a "revolutionary upheaval that is unparalleled in its modern history."

He said the progress achieved so far in fighting Islamic State militants in Iraq has been "inadequate" and suggested the U.S. increase support to Iraqi security forces and Sunni tribal and Kurdish fighters. In Syria, he recommended the U.S. take a harder stance against President Bashar Assad. He supported setting up enclaves protected by coalition airpower where moderate Sunnis could be supported, civilians could find refuge and additional forces could be trained.

"The Middle East is not part of the world that plays by Las Vegas rules: What happens in the Middle East is not going to stay in the Middle East," he warned.

Petraeus began his testimony, however, with an extraordinary apology for events stemming from his personal life. He was director of the CIA from September 2011 to November 2012, when he resigned after acknowledging an affair with Broadwell, a married U.S. Army reserve officer who met Petraeus while researching a book about his wartime leadership in Iraq and Afghanistan.

"Four years ago, I made a serious mistake — one that brought discredit on me and pain to those closest to me," Petraeus said. "It was a violation of the trust placed in me and a breach of the values to which I had been committed throughout my life."

"There is nothing I can do to undo what I did. I can only say again how sorry I am to those I let down and then strive to go forward with a greater sense of humility and purpose, and with gratitude to those who stood with me during a very difficult chapter in my life."

The retired four-star general was sentenced to two years of probation and fined $100,000 for unauthorized removal and retention of classified information he shared with Broadwell.

Before becoming CIA director, Petraeus commanded U.S. and coalition forces in Iraq and Afghanistan. On Iraq, Petraeus told the lawmakers that while there have been significant accomplishments in the fight against IS, "We are not where we should be at this point."

In addition to increasing support for local fighters, he suggested embedding U.S. advisers down to the brigade headquarters level for Iraqi fighting forces; exploring the use of air controllers with select Iraqi units to coordinate coalition airstrikes; and examining whether U.S. rules of military engagement for precision airstrikes are too restrictive.

Petraeus said, however, that the U.S. should not allow its forces to take over Iraqi units. "I would not, for example, embed U.S. personnel at the Iraqi battalion level, nor would I support clearance operations before a viable force is available," he said.

He called Syria a "geopolitical Chernobyl — spewing instability and extremism over the region and the rest of the world."

"Like a nuclear disaster, the fallout from the meltdown of Syria threatens to be with us for decades, and the longer it is permitted to continue, the more severe the damage will be."

He said the U.S. is no closer today to having a moderate Sunni Arab ground force than a year ago.

Last week, Gen. Lloyd Austin, commander of U.S. Central Command, which oversees the war effort, told the committee that only a handful of U.S.-trained Syrian rebels are still on the battlefield fighting the militants. The four or five fighters still engaged in the campaign is astonishingly short of the U.S. goal to train and equip 5,400 rebels a year at a cost of $500 million.

"The central problem in Syria is that Sunni Arabs will not be willing partners against the Islamic State unless we commit to protect them and the broader Syrian population against all enemies, not just ISIS," Petraeus said using an acronym for the militant group. "That means protecting them from the unrestricted warfare being waged against them by Bashar Assad, especially by his air force and its use of barrel bombs."

He suggested that the U.S. tell Assad that if he continues to use barrel bombs, the U.S. will stop the Syrian air force from flying.

"We have that capability," he said. "It would demonstrate that the United States is willing to stand against Assad and it would show the Syrian people that we can do what the Islamic State cannot — provide them with a measure of protection."

At the same time, Petraeus warned against rushing to oust Assad without knowing who would fill the resulting political vacuum in the country.