At least 27 people died in a car bombing in rebel-held northwestern Syria as the United States stepped up its calls for a peace conference between the rebels and the Assad regime.
Also Monday, suspected jihadists freed four out of seven aid workers kidnapped in the increasingly volatile region.
The ICRC has not commented on the nationality of those abducted, though it has said most of the group were Syrian. There has been no claim of responsibility.
The United Nations said, meanwhile, that "the race is on" to make sure Syria keeps to deadlines to destroy its chemical weapons arsenal under the terms of a UN Security Council resolution.
The blast in the town of Darkush killed at least 27 people, including three children and a woman, the Syrian Observatory for Human Rights said, adding the death toll could rise because many of the wounded were in serious condition.
Activists said the blast targeted the market area of the town, which is a few kilometres (miles) from the border with Turkey, on the Orontes River.
Four of the kidnapped aid workers were meanwhile freed, International Committee of the Red Cross spokesman Ewan Watson said, adding the organisation was awaiting further information about the others.
The Observatory said an Al-Qaeda-linked rebel group -- the Islamic State of Iraq and the Levant (ISIL) -- kidnapped the six Red Cross staff and a volunteer from the Syrian Red Crescent.
The Red Cross, one of the few organisations able to deliver aid nationwide, said the team had travelled to Idlib on October 10 to assess the situation at health facilities and deliver aid.
"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," Watson had earlier told Swiss radio.
Rebels control large swathes of Idlib, and kidnappings have become increasingly common in rebel-held parts of Syria, targeting both journalists and aid workers.
'Urgent' need for peace talks
The bombing and abductions came as the United States stepped up its calls for a peace conference between President Bashar al-Assad's regime and its Western-backed opponents, who are divided over the initiative.
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The United States said there was an "urgent" need to set a date for so-called Geneva 2 peace talks, despite a leading opposition group's rejection of the process.
"We believe that it is urgent to set a date to convene the conference and work toward a new Syria," US Secretary of State John Kerry said after meeting UN-Arab League envoy Lakhdar Brahimi in London.
"There has to be a transition government," he said. "This will require all the parties to come together in good faith."
Russian Foreign Minister Sergei Lavrov urged Washington to bring the opposition to the talks proposed for mid-November.
His call came a day after the Syrian National Council -- a key component of the National Coalition recognised by most Arab and Western governments -- ruled out attending any Geneva peace talks, and said it would quit the umbrella group if it participated.
The United States threatened strikes in response to August 21 sarin attacks that killed hundreds of people on the outskirts of Damascus, but the military action was averted by a US-Russian deal under which Syria is turning over its chemical arsenal for destruction.
SNC chief George Sabra said his group would boycott the Geneva talks because the international community had failed to punish the gas attacks or address the plight of civilians in besieged areas.
The United Nations said Monday the Syrian regime was providing "good cooperation" in the bid to destroy the country's chemical weapons by mid-2014.
However Medical charity Doctors Without Borders (MSF) called Tuesday on Syria's warring groups and the international community to make aiding needy civilians as urgent a priority as dealing with chemical weapons.
"Influential countries gathered around a table, thrashed out an agreement on chemical weapons and put it into practice. They have shown it can be done, so where are the efforts to repeat this success with the burning question of access for humanitarian aid?" said MSF General Director Christopher Stokes in a statement.
The comments came as the Chemical Weapons Convention came into force for Syria.
The United Nations and the Organisation for the Prohibition of Chemical Weapons have been working in Syria since October 1 to eradicate the country's banned arms.
The OPCW says Syria's war is already holding up the work and has appealed for local truces to get access to weapons sites.
The appeal from MSF came four days after the OPCW won the Nobel Peace Prize for its work to rid the world of the devastating weapons.