A Free Syrian Army fighter takes position behind sandbags on the front lines of Wadi Al-Dayf camp in northwestern Syria. (Khalil Ashawi/Reuters)

The Pentagon has deployed military assessment teams to Saudi Arabia in advance of its training of moderate Syrian rebels to fight the Islamic State group terrorizing Syria and Iraq, but the chairman of the Joint Chiefs of Staff cautioned Friday that it will take time for the effort to gel.

“We have to do it right, not fast,” said the chairman, Army Gen. Martin E. Dempsey. “They have to have military leaders that bind them together. They have to … have a political structure into which they can hook, and therefore be responsive to. And that’s gonna take some time.”

The training is expected to be a centerpiece of the expanded U.S.-led effort to fight the Islamic State in Syria. U.S. officials plan to initially train 5,000 rebels in Syria, providing a ground force that can be paired with the airstrike campaign that the United States and partner nations launched there this week. But the effort could eventually expand beyond that, Dempsey said.

“Five thousand’s never been the end state,” the general said of the number of trained rebels he believes are needed in Syria. An estimated 12,000 to 15,000 would be needed “to recapture lost territory in eastern Syria.”

Hagel said the United States is in the process of setting up a vetting system for the rebels to make sure they don’t pose a threat to the U.S. troops who would train them — a significant concern raised by members and Congress and others who question the legitimacy of the Free Syrian Army and other rebel groups deemed moderate by the United States. It isn’t clear when that vetting process will start, however, or who Washington will consider the rebel leaders.

“We’re not gonna instruct them as to who their leaders are,” Hagel said. “They’ll make their own decision on who their leaders are.”

Dempsey and Hagel spoke to reporters Friday in their first extended news conference since the United States began dropping bombs on Syria on Monday night Eastern Daylight Time. Hagel said the United States and its Arab partners have launched 43 airstrikes in Syria thus far, and the Pentagon has carried out more than 200 airstrikes on militant targets in Iraq since they were approved by President Obama on Aug. 7.

The coalition dropping bombs will expand soon. In Syria, it includes Saudi Arabia, Jordan, the United Arab Emirates and Bahrain, with Qatar in a supporting role so far, Pentagon officials said. In Iraq, France launched its first airstrikes on Thursday, and Britain’s Parliament voted on Friday to join the fight. The Netherlands, Belgium and Australia also have joined the campaign.

Turkish President Recep Tayyip Erdogan said this week that his country, which has faced increasing tension on its Syrian border, will play a role in the fight against the Islamic State. It is unclear what that will entail, however. Erdogan said Friday that a no-fly zone should be created over Syria to protect it from attacks ordered by the Syrian President Bashar al-Assad’s government. Turkey also is pushing for Western help to develop a military buffer zone along its border with Syria, but U.S. officials have been non-committal so far.

“Look, a buffer zone might, at some point, might become a possibility, but that’s not part of our campaign plan presently,” Dempsey said.