The five staffers were taken "allegedly for questioning" from a Doctors Without Borders house in northern Syria, and have been out of contact since Thursday evening, said Michael Goldfarb, a spokesman for the aid group. He did not say whether the missing staffers had been taken by government forces or rebels fighting to overthrow President Bashar Assad, and refused to give further details out of concern for the missing workers' safety.
Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights, said that members of the al-Qaida-linked Islamic State of Iraq and the Levant had stormed a hospital in the northwestern province of Latakia and taken all of its doctors to an unknown location. Others were taken from their homes by ISIL members, he added.
Abdurrahman said it was not clear whether the Doctors Without Borders members were among those taken in Latakia.
Opposition-held areas of northern and eastern Syria have seen a wave of kidnappings over the past six months that has targeted journalists, aid workers and activists. Al-Qaida-linked rebel factions are suspected of being behind many of the abductions.
In October, several members of the International Committee of the Red Cross were briefly abducted in northwestern Syria. Many Syrian activists have fled the country after threats by ISIL and the killing of a number of citizen journalists.
Also Friday, activists reported heavy clashes between Syrian opposition fighters and ISIL members in the northern provinces of Aleppo of Idlib.
More than 130,000 people have been killed so far in the war, now in its third year, according to the Observatory. The group closely monitors the violence in Syria through a network of activists across the country. The U.N. said in July that 100,000 Syrians have been killed, and has not updated that figure since.
Millions of Syrians have been uprooted from their homes because of the fighting.
The crisis began as an uprising against President Bashar Assad's government but later turned into a civil war. Over the past years, clashes between rival opposition groups became common in rebel-held areas.
The Observatory said Friday's fighting was concentrated in the city of Aleppo and the nearby town of Atareb. Fighting between rebel factions and ISIL, which has many foreign fighters in its ranks, has been common in areas under opposition control.
The Aleppo Media Center reported clashes between the ISIL and the Mujahedeen Army in rebel-held western parts of Aleppo.
Tension has been rising between rebel groups and ISIL of late, especially after Tuesday's killing of a senior rebel by the al-Qaida-linked group. The Observatory said the slain rebel, doctor Hussein Suleiman, was found dead after being shot several times and his ear was cut "with a sharp object."
The newly created Islamic Front, an umbrella group of powerful, mostly ultra-conservative Islamic fighters, issued a statement ordering ISIL to hand over Suleiman's killers so they can stand trial. Clashes later erupted between the groups.
A video released by activists showed dozens of people marching in Aleppo and chanting "Free Syrian Army forever, against ISIL and Assad."
The video appeared genuine and corresponded to other AP reporting of the events depicted.
In Damascus, meanwhile, the Minister of Oil and Mineral Resources, Suleiman al-Abbas, said "terrorists" attacked a gas pipeline in the area of al-Baitaryeh outside Damascus, causing a massive fire and a power outage in the capital and southern Syria. He said terrorists also targeted a gas line that supplies the Banias Power Station in the area of al-Zara outside the central city of Homs.
The government refers to those trying to topple Assad as terrorists.
Associated Press writer Albert Aji in Damascus, Syria, contributed to this report.