BEIRUT — In a brazen assault near the Syrian capital, Islamic State militants abducted 300 cement workers and contractors from their workplace northeast of Damascus on Thursday even as fighting against the extremist group raged in the country's north ahead of a new round of peace talks.

In a blow to the Islamic militants, however, rebel fighters wrested control of a northern border town, potentially undermining Islamic State supply lines across the border with Turkey and endangering one of its most important strongholds in Aleppo province.

The U.N. special envoy for Syria, meanwhile, said the next round of peace talks in Geneva was expected to start next week, around April 13, and would focus on a political process that he hopes will lead to a “concrete or real beginning of a political transition.”

Speaking to reporters in Geneva, Staffan de Mistura said he was encouraged by the fact that a partial cease-fire has largely held since going into effect Feb. 27, despite a series of worrisome incidents.

State TV said Thursday's mass abduction of workers from the al-Badia Cement Company took place in Dumeir, about 28 miles northeast of Damascus, where militants launched a surprise attack against government forces earlier this week. The state-run news agency SANA quoted a source in the company as saying there had been no success in efforts to establish contact with the workers.

There was no formal claim of responsibility for the kidnapping, but the Islamic State-linked Aamaq media agency posted a video showing the deserted cement factory, located near a military air base. The video showed what appeared to be a Syrian soldier lying on the ground, apparently dead. One militant is seen driving a truck, towing away a fork lift.

Islam Alloush, a spokesman for the Army of Islam rebel group, which has a strong presence in Dumeir, said Islamic militants attacked five targets in the town, including other insurgents' positions near the airport. They also seized control of the factory, kidnapping hundreds of its workers.

Mass abductions have taken place on occasion in Syria during the country's civil war, now in its sixth year.

The abduction came as fighting raged in northern Syria, where Syrian opposition fighters made quick advances on Islamic State strongholds, including al-Rai along the border with Turkey.

Opposition activists and a rebel group said Free Syrian Army factions took over al-Rai, in northern Aleppo province, potentially severing one of the extremist group's most important crossings with Turkey.

FSA Division 99 said on its Facebook page that its fighters along with other groups were deployed inside and combing the town and had taken down the Islamic State flag. The Islamic State-linked media agency Aamaq said at least 14 factions were fighting against the militant group in al-Rai.

The Britain-based Syrian Observatory for Human Rights, which monitors the Syria conflict, and the Local Coordination Committees activist group also said opposition forces were inside the town, driving out dozens of Islamic State fighters.

If al-Rai falls, the rebels will be able to challenge the Islamic State's hold on al-Bab, the group's stronghold in Aleppo, activist Bahaa al-Halabi said.

In Geneva, de Mistura told reporters he was embarking on a Mideast tour that will take him to Damascus as well as the Iranian capital, Tehran, and possibly Amman, Jordan, in search of an understanding about what could be a framework of a political transition.

Two rounds of “proximity talks” involving Syrian opposition and government representatives in Geneva have ended without any progress on ways to end the war in Syria.

De Mistura said that he would hold talks with senior Syrian officials in Damascus but that he had not requested a meeting with President Bashar Assad.