The fate of three other Red Cross workers who were also seized Sunday in the northwestern Idlib province remained unclear, the International Committee of the Red Cross said.
Syrian opposition activists said the seven aid workers were taken at a rebel checkpoint outside the town of Saraqeb, manned by an al-Qaida-affiliate, the Islamic State in Iraq and the Levant. There was no claim of responsibility.
About two dozen miles away, near Turkey, a car bomb went off in the market of the town of Darkoush on Monday, while it was crowded with people shopping for the four-day Muslim holiday of Eid al-Adha holiday. The blast set cars on fire and sent people running.
The Britain-based Syrian Observatory for Human Rights said 27 people were killed, while another activist group, the Local Coordination Committees, put the death toll at 15.
It was not clear who carried out the bombing and why they attacked a civilian target in a rebel-held area. Syria's conflict has seen an increasing use of car bombings, but most have been carried out against regime targets, usually by jihadi fighters among rebels.
Meanwhile, Syria became a full member of the Organization for the Prohibition of Chemical Weapons on Monday, in another step toward eliminating its chemical weapons stockpile by mid-2014.
The mission is overseen by the OPCW and the United Nations. The joint team has inspected five of at least 20 sites in the past two weeks, according to the OPCW chief.
Ahmet Uzumcu signaled that the team of 60 OPCW inspectors and U.N. staff is encountering difficulties. He was quoted as saying that one abandoned site was in rebel-held territory and that in other cases, routes went through opposition-controlled areas, preventing access because rebels have not promised cooperation.
"They (the areas) change hands from one day to another, which is why we appeal to all sides in Syria to support this mission, to be cooperative and not render this mission more difficult. It's already challenging," he told the BBC.
The OPCW won the Nobel Peace Prize last week, in a strong endorsement of its Syria mission.
U.N. Secretary-General Ban Ki-moon, meanwhile, selected Sigrid Kaag, a Middle East expert from the Netherlands, to head the joint OPCW-U.N. team in Syria, U.N. diplomats said. Kaag is an assistant administrator of the U.N. Development Program and speaks Arabic, said the diplomats who spoke on condition of anonymity ahead of a public announcement.
The push to eliminate Syria's stockpile of about 1,000 metric tons of blistering and nerve agents stems from an Aug. 21 chemical weapons attack on opposition-held suburbs of Damascus. Hundreds were killed, including many children. The West says the Syrian government was responsible, while Damascus blames the rebels.
Syrian President Bashar Assad said Monday that his country stopped manufacturing chemical agents in 1997 because they became an "outdated deterrent." He said Syria has since concentrated on its missile capabilities.
Damascus is believed to have thousands of long-range missiles that can reach targets almost anywhere inside Israel, its archenemy.
"Developing Syria's missile deterrent force that can be used from the first moments of war ended the necessity of chemical weapons," Assad was quoted as saying in the Lebanese Al-Akbar newspaper.
Nonetheless, Assad said, Syria is suffering a "moral and political loss" in handing over its chemical weapons.
Asked about the OPCW's Nobel prize, Assad attempted an apparent joke, saying, "this prize should have been mine."
More than 100,000 people have been killed and millions displaced since the Syria conflict erupted in March 2011, with a popular uprising that escalated into a civil war. The country has turned into a patchwork of regime- and rebel-held areas. Assad's political opponents are divided, while Islamic extremists have emerged as dominant in many rebel areas.
Despite the fractured nature of the opposition, U.S. Secretary of State John Kerry said talks on a political transition must begin by mid-November, as envisioned by the U.N.
Kerry said Syria urgently needs a transitional government, but that Assad "has lost the legitimacy to be able to be a cohesive force that could bring people together."
It's uncertain if the Syrian political opposition will attend.
Opposition figure George Sabra said a final decision of the Syrian National Council, the main Western-backed umbrella group, is expected at a conference starting Oct. 24. Sabra's group, the largest in the council, won't attend transition talks, he said.
The opposition wants Assad to step down first. It has also expressed anger over the chemical weapons deal, in which the regime is treated as a partner. "Unfortunately, they let the criminal escape from punishment," Sabra said.
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