Rebels suggested that Russian jets carried out the attack, which also injured at least 40.
Amnesty International documented 24 cases of abduction in Aleppo and Idlib governorates.
When a rebel fighter who is a father of four is kidnapped by ISIS, the family flees to Germany.
Syrian state news agency claims militant group used mustard gas on troops at eastern airbase.
An opposition representative said the delegation might turn up if their demands were swiftly met, but the chances appeared slim.
The rebels want all sides in Syria's civil war to allow humanitarian access to those in need and cease attacks on civilians.
Jaysh al Islam, a Syrian insurgent group, welcomes Saudi Arabia's cutting ties with Iran due to Tehran backing of Shiite militias.
Al-Shadadi is a key logistics hub for the group.
It is hoped that the factions can form a united opposition ahead of January peace talks with the government.
A top Free Syrian Army commander describes how the Russians bolster Assad's forces.
Kerry also said the war in Syria could be weeks away from a major breakthrough.
Chemical weapons experts say mustard gas was used in a Syrian town where ISIS was battling a rebel group.
Putin spoke a day after Assad made a surprise visit to Moscow for talks with the Russian leader.
The Free Syrian Army rebel group confirmed the death of its chief of staff, Basil Zamo, formerly a captain in the Syrian military.
Government troops and their allies backed by Russian jets have launched an offensive against rebels near Aleppo.
Gains by ISIS north of Aleppo would threaten the supply lines of rival rebels inside the city.