In pictures: Race against time in devastated Turkey, Syria

Rescuers were scouring debris on Friday nearly 100 hours after a massive 7.8 earthquake hit Turkey and Syria, killing nearly 22,000 people so far in one of the region's worst disasters for a century.

The first UN aid deliveries arrived on Thursday in northwestern Syrian rebel-held zones, but chances of finding survivors have dimmed since the passing of the three-day mark that experts consider a critical period to save lives.

Monday's quake was the largest Turkey has seen since 1939, when 33,000 people died in the eastern Erzincan province.

Officials and medics said 17,674 people had died in Turkey and 3,377 in Syria from Monday's tremor, bringing the confirmed total to 21,051.

Experts fear the number will continue to rise sharply.

In addition to a staggering human toll, the quake's economic cost appears likely to exceed $2 billion and could reach $4 billion or more, Fitch Ratings said.