GAZIANTEP, Turkey — Seventy-two Syrian rebel groups on Saturday announced a new coalition to battle the government of President Bashar Assad. But hopes that moderate rebels would dominate the meeting were dashed when extremists gained more of the 17 executive positions than had been expected.

Col. Muhammad Hallak, who represented a moderate faction attending the three-day organizational meeting, accused Islamists, especially Ahrar al Sham, a group with links to al-Qaida and its Syrian affiliate, the Nusra Front, of capturing more positions than its influence in the rebellion deserved.

A review of the names indicates that moderates hold only six or seven of the 17 positions.

Hallak also expressed skepticism toward the October document on which the new group, the Revolutionary Command Council, is based, saying it was written to ensure an Islamist government after Assad is toppled.

The announcement of the new umbrella group comes as moderate rebels have lost territory to the Nusra Front, especially in Idlib province, where groups associated with the U.S.-backed Free Syrian Army coalition had been in control.

“The covenant itself doesn’t mention the idea of free elections and most of the groups represented in the executive office don’t believe in the original democratic values of the revolution,” Hallak said.

On Friday, as the groups were meeting here, the Nusra Front stormed the bases of two moderate rebel groups in Syria’s north: the Ansar Brigades in Idlib and the Haqq Front in Hama.

The two groups, both of which were receiving U.S. support through a covert CIA program, surrendered to the Nusra Front, delivered their weapons to Ahrar al Sham and returned to their homes. Some of the leaders escaped to Turkey, fearing arrest or revenge from the Nusra Front.

With those attacks, the Nusra Front completed the elimination of FSA groups from Idlib and leaves just one group in Hama province receiving U.S. support. The Nusra Front now controls all the resupply routes to Turkey.

On Saturday, the newly elected head of the Revolutionary Command Council, retired judge Kais al Shaiek, said that the council would not replace the Syrian Opposition Coalition, the rebel organization that the Obama administration once recognized as the sole representative of the Syrian people. But he pointedly noted that his council would represent the interests of people inside Syria, a reference to accusations that the opposition coalition has little support among rebel groups doing the fighting.

Shaiek said, however, that the new council held its meeting in Turkey because it feared attacks from the government.