BRUSSELS — Moderate Syrian opposition forces claim they are strengthening their positions against extremists in northern and southern Syria as the Russian military withdraws and a fragile cease-fire takes hold with the regime of President Bashar al-Assad.

Two rebel commanders described the recent battlefield gains against the Islamic State and Jabhat al-Nusra in telephone interviews Saturday. Their claims couldn’t be verified, and it’s too early to say whether the moderate opposition is truly expanding its support in rebel-held areas, after several years of decline in the face of attacks by the extremists and Assad’s army.

Advocates of the Syrian peace process have hoped for this chain of events, where moderate forces — freed from devastating attacks by Russia and Assad’s army — could mobilize civilian opposition to the extremists. What’s crucial now, the rebel commanders said, is that they receive humanitarian assistance quickly so that they can provide services and rudimentary local governance in areas where they are gaining control.

Syria-watchers should be cautious about such reports. This isn’t the first time that moderate, U.S.-backed commanders have claimed success — only to be reversed by the double-whammy of the regime and the jihadists. But if the reports are true, the situation is at least moving in the right direction.

The rebel commanders spoke from Geneva, where they arrived recently from Syria to join in U.N.-sponsored discussions about political transition that are backed by the United States and Russia. The two spoke through a translator, in a conversation that was arranged by the Higher Negotiation Committee, the umbrella group for the Syrian opposition.

Both commanders are members of opposition groups that have been armed and trained by the United States, Jordan and other partners. They carry modern, U.S.-made anti-tank missiles and other weapons that have given them more heft on the battlefield than some other opposition groups.

Maj. Abu Usama al-Joulani, the leader of the Syrian Revolutionaries Front in al-Quneitra, which he said has about 7,000 fighters, described recent events in southern Syria. He said that after the cease-fire was announced in late February, the Islamic State tried to activate “sleeper cells” in the south, in the hope of expanding extremist support there.

But Joulani said moderate forces had arrested about 80 suspected extremists in the Daraa area and blocked their effort to expand the Islamic State’s network. He said that in other fighting in the south, his group had killed about 30 Islamic State fighters and captured about 10. He said the biggest town liberated from the Islamic State in recent fighting was Ankhal, about five miles north of Daraa.

“Our priority was to clear the area and work with other fighters” against the extremists, Joulani said. He said that until President Vladimir Putin announced a military withdrawal a week ago, the Russians and the Assad regime had regularly attacked his forces. “The cease-fire is working very well,” allowing the U.S.-backed rebels to expand their operations against extremists groups, he said.

Joulani said his goal now was to organize an elite “special operations force” of about 500 Syrian rebels that could press the campaign against the extremists in the south.

A rebel commander named Eyad al-Shamsi described the situation in northern Syria. He said that since the cease-fire, his group, which he called the Asala and Tanmiya Front, and other moderate opposition groups had been able to capture about 10 villages north of Aleppo that had been held by the Islamic State. The cleared villages included Doudyan and Mregel, both east of Azaz and just south of the Turkish border. He said his group, with several thousand fighters, has been partially armed by the United States

This area around Azaz has been the scene of heavy fighting for months, and Islamic State fighters still maintain supply lines all the way east to their capital in Raqqa, so they can quickly counterattack. What’s new is that Shamsi’s group is Sunni; most of the successes against the Islamic State in the north have been won by Syrian Kurdish fighters from the YPG, rather than Sunni rebels.

Shamsi said there have also been recent setbacks in northwest Syria for Jabhat al-Nusra, an extremist group that’s allied with Al-Qaeda. He said that after the cease-fire, the Jabhat al-Nusra extremists had tried to tighten their control in Idlib province, and had attacked Syrian civilians who were carrying banners to mark the fifth anniversary of the revolution to topple Assad’s regime. Jabhat al-Nusra also fought with a moderate group known as “Division 13,” further alienating some civilians, according to Shamsi.

“When the cease-fire started, Jabhat al-Nusra lost its legitimacy in the eyes of the people,” Shamsi claimed. “People felt they were going to have normal life,” and were angered by Jabhat al-Nusra’s attempt to impose its extremist rules. Syrians in the town of Maarrat al-Numan, south of Idlib city, organized to expel the Jabhat al-Nusra fighters, Shamsi said. “When the people had a chance for stability, they took it,” he argued.

These developments inside Syria, if they are confirmed, are consistent with the advice offered by a group of prominent Syria experts who spoke Friday night at the Brussels Forum, organized by the German Marshall Fund, of which I’m a trustee. The experts included Samir al-Taqi, a Syrian exile who heads the Orient Research Center in Dubai, Borge Brende, the foreign minister of Norway, Jean-Marie Guehenno, the lead of the International Crisis Group, and Nancy Lindborg, president of the U.S. Institute of Peace.

Though their specific comments were off the record, there appeared to be a loose consensus within this group on several points: Syrians must play a greater role in the political transition, rather than simply accepting dictation from Moscow and Washington; a role for Syrian Kurds must be found, but not so potent that it threatens the interests of Turkey; moves against the Islamic State and Jabhat al-Nusra must involve Syrian Sunni fighters, rather than simply an air campaign by the United States and its partners; and western humanitarian aid and mediation is essential to consolidate the cease-fire.

On all these points, Saturday’s reports from the two Syrian commanders are encouraging. Syria is a recurring lesson in the failure of good intentions, but there’s at least a more visible path forward now, with the cease-fire still holding after more than three weeks.