More than 21,000 people have been killed and thousands more injured as efforts continue for a fourth day in freezing conditions to save those still trapped under rubble.

Officials and medics said Thursday that 17,674 people had died in Turkey and 3,377 in Syria, bringing the confirmed total to 21,051.

Initial rescue efforts were hampered by a winter storm that covered major roads in ice and snow and left three key airports in the area inoperable, complicating deliveries of vital aid.

Survivors are still being pulled from collapsed buildings, though disaster experts warn that the chances of saving lives drop sharply after 72 hours.