Efforts continued Monday to free three aid workers from the International Committee of the Red Cross who were kidnapped in Syria, after four fellow abductees were released.
"We are still doing our utmost to have our 3 other colleagues back safe & sound," the ICRC's director general Yves Daccord said on his Twitter feed.
No comment was available from the agency on a statement by the Syrian Observatory of Human Rights that the workers were kidnapped by the Islamic State of Iraq and the Levant, an Al-Qaeda-linked rebel group.
Rebels control large swathes of Idlib province, where six ICRC staff and a volunteer from the Syrian Arab Red Crescent were abducted by gunmen Sunday.
Kidnappings have become increasingly common in rebel-held parts of Syria, targeting both journalists and aid workers.
"Three ICRC colleagues and the Syrian Arab Red Crescent volunteer have been released and are safe and sound. We are waiting for further information about the other three colleagues," ICRC spokesman Ewan Watson told AFP.
No group has claimed responsibility for the kidnapping, which took place as the aid workers' convoy was driving back to Damascus after a four-day medical aid mission in Idlib.
The ICRC has not revealed details on their nationalities, although it earlier said that most of the group were Syrian.
Despite the kidnapping, which underlined the risks facing aid workers in Syria, the ICRC has vowed to continue its work in the war-torn country.
"We are completely committed to supporting the Syrian population in this difficult moment," Watson told Swiss public radio.
But the aid organisation said it was reviewing its security.
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"We don't have any intention of stopping our activities in Syria, but of course this situation makes us reflect and take a close look at our operations because in the end, we will not be able to work and help the Syrian population without having security for our personnel," Watson said.
"We are worried that these types of incidents will prevent us from having as large of an access in the future and from carrying out our humanitarian work," he added.
Last year, the ICRC halted operations in parts of Pakistan following the kidnapping and murder of a British employee there.
The ICRC has some 30 expatriate staff in Syria, along with 120 local employees.
They work hand in hand with the Syrian Arab Red Crescent, one of the few organisations able to deliver aid nationwide.
Security is a constant concern as aid workers go back and forth across the often fluid front lines in the war between a range of rebel groups and forces loyal to President Bashar al-Assad.
Twenty-two Red Crescent volunteers have been killed since the conflict erupted in March 2011, the ICRC's Damascus spokesman Simon Schorno told AFP, adding that he did not immediately have a figure for the number active in the field.
The United Nations has 4,800 people working in Syria, said Jens Laerke, spokesman for its Office for the Coordination of Humanitarian Affairs, the vast majority of them Syrians.
According to the specialist website aidworkersecurity.org, whose data runs to September 4, a total of 39 Syrian aid workers have been killed or wounded since the war began.
Two foreign staff have also died, while three German aid workers this year escaped several months after being kidnapped in Idlib.
Syria's war has claimed more than 115,000 lives, driven over two million people out of the country and left millions more within its borders reliant on aid to survive.