Officials have said a Syrian Arab Red Crescent volunteer and three of six Red Cross staffers taken hostage in Syria are free and safe.

Robert Mardini, the International Committee of the Red Cross' head of operations for the Middle East, said on Twitter that the four were released "safe and sound."

The ICRC's Geneva office confirmed the tweet was accurate, but provided no more details Monday.

Gunmen abducted the seven on Sunday after stopping their convoy in northwestern Syria.

The incident highlights an increasing problem within Syria as kidnappings are on the rise. The growing risk of kidnappings means that foreign staff have reduced their presence within the country.

The security situation has got much worse in recent months, especially in August, given the rise of the influence of extremist groups directly linked to al Qaeda," said Jitka Korenkova, who works for the Czech People in Need, one of the few aid groups working in Aleppo city, in north Syria.

Growing security issues

Hours after the kidnapping of aid workers a car bomb exploded in the town of Darkoush, a rebel-held town in Idlib province, killing 39 people and injuring dozens of others.

The UK-based Syrian Observatory for Human Rights said the toll was expected to rise due to the large number of people who suffered serious injuries.

A video posted by activists online showed the aftermath of the blast, with at least one car ablaze and the ground around it covered in smoking embers.

A second video showed residents carrying bodies on makeshift stretchers and extensive damage to buildings around the blast site.

"The area is controlled by rebels; government forces were pushed out months ago. There are no claims of responsibility," said Al Jazeera's Zeina Khodr, reporting from Beirut, the capital of neighbouring Lebanon. "There's still a lot of insecurity in the north, even though most of that area is held by the rebels."