U.S. Secretary of State John Kerry and the U.N.-Arab League envoy for Syria say an international conference to set up a Syrian transitional government must be organized urgently and held as soon as possible.

After talks Monday in London with Lakdhar Brahimi, Kerry said that Syrian President Bashar al-Assad "has lost the legitimacy to be a cohesive force that could bring people together."

He called for a transition government in Syria "to permit the possibility of peace," saying it was imperative to get the so-called "Geneva II" conference organized by a mid-November target the United Nations has set.

Brahimi agreed, saying a precise date for the peace talks must be set "very soon."

A key opposition group, the Syrian National Council, said Sunday it would not take part and criticized the international community's response to the fighting.

Meanwhile, activists said a car bomb exploded Monday in the northern town of Darkoush near the Turkish border, killing at least 20 people and wounding dozens of others. The Syrian Observatory for Human Rights said the blast hit the town's market. There has been no claim of responsibility for the blast.

Also Monday, the International Committee of the Red Cross said it will not stop its work in Syria after gunmen in the country's north kidnapped six Red Cross aid workers and a member of the Syrian Arab Red Crescent.

Spokesmen for the aid group said they will review safety and security measures after the kidnapping, which happened Sunday morning in Idlib province where the workers had been on a medical assistance mission.

Kidnappings have become increasingly common in northern Syria, where rebel factions fighting the Syrian government have captured large areas.