(CNN) -- Defense Secretary Leon Panetta arrived in Tunisia late Sunday for the first leg of a five-day, four-nation trip to North Africa and the Middle East.
Besides Tunisia, he is expected to visit Egypt, Israel and Jordan, and to meet with officials at each stop. High on the agenda is Syria.
Speaking aboard a military aircraft before touching down in Tunis, Panetta addressed the crisis in Syria, saying it's no longer a question of whether the regime of Syrian President Bashar al-Assad will fall, but when.
"The more violence he engages in, the more he makes the case that that regime is coming to the end," the secretary told reporters. "Aleppo is another example of indiscriminate violence against his own people that just makes very clear that he has lost all credibility to lead that country."
Panetta was referring to the ongoing battle for Syria's largest city.
"If they continue this kind of tragic attack on their own people in Aleppo, I think it ultimately will be a nail in Assad's coffin," the secretary said.
In Jordan, Panetta expects to discuss what's happening in Syria and the impact it could have on regional stability. The United States is working closely with Jordan, as well as with international and nongovernmental groups, to provide humanitarian assistance to those affected by the violence, he said.
The secretary will arrive in Israel just days after presumptive GOP nominee Mitt Romney visited Jerusalem and pledged to support "any and all measures" to keep Iran from developing a nuclear bomb.
Panetta declined to comment on a question about Romeny's trip, but offered support for the position of President Barack Obama.
"I think the president has made clear that the United States stands with Israel and the international community in making very clear that we will not allow Iran to develop a nuclear weapon," he said. "This hasn't been easy. It's been tough. But I think the fact is that when the United States, Israel and the international community remain unified in our position against Iran that that's the best way to convince Iran to pull back from what they are doing and to abide by international rules and regulations."
Iran insists its nuclear program is exclusively for peaceful purposes.
While in Egypt, Panetta will meet with newly-elected President Mohamed Morsy. He was sworn into office late last month, but the powerful Supreme Council of the Armed Forces (SCAF) wields legislative power, having ordered the dissolution of parliament after the country's highest court ruled that it had been elected under invalid laws.
Morsy tried to call it back into session, but the court reaffirmed its decision, so the military council retains lawmaking powers until a new parliament is sworn in near the end of the year.
"My message has been consistent. We strongly support an orderly, peaceful, legitimate transition to a democratic system of government. The SCAF's support for a secure, free and fair presidential election last month was a critical step in this process," said Panetta, adding that he will urge the government to complete the transition to full civilian rule.