Syrian rebels claimed to have seized a key southwestern city close to the border with Jordan as on Saturday they pressed on with a lightning-fast offensive that threatens to further destabilize a region already convulsed by war.

The capture of Daraa would mark a symbolic and strategic victory for the rebels as the place where pro-democracy protests in 2011 sparked the country’s ongoing civil war, while it is also an important crossing on the border.

“The city of Daraa is completely…