Five staffers of the international aid organization Doctors Without Borders have been taken in for questioning in northern Syria, the group said Friday.

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 al-Assad, and refused to give further details out of concern for the missing workers' safety.

Karin Ekholm, a spokeswoman for the Swedish branch of the organization, said those seized were Swedish, Danish, Swiss, Belgian and Peruvian nationals.

"We are doing all we can to do re-establish contact with our colleagues," the Danish branch said in a statement.

Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights, said that members of the al-Qaeda-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.

Wave of kidnappings

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-Qaeda-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 UN 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 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 government refers to those trying to topple Assad as terrorists.