GENEVA - The World Health Organization (WHO) expects a significant jump in the death toll following a major earthquake and its aftershocks in southern Turkey and northwestern Syria that reduced many buildings to rubble.

The magnitude 7.8 quake, which rattled southern Turkey early on Monday, was the worst to hit the country this century, killing more than 900 people there and about 550 across the border in Syria, according to officials.

It was followed hours later by another large temblor of magnitude 7.7.

"I think we can expect the death toll to increase significantly," Rick Brennan, the WHO's regional emergency director for the Eastern Mediterranean, told Reuters.

"There's been a lot of building collapses and it will increase more significantly around the epicentre of the earthquake."

Brennan said WHO was boosting its staff in the southern Turkish city of Gaziantep, the epicentre of the earthquake, and exploring its options to send emergency medical teams to the area. He said rescue efforts were being hampered by aftershocks from the initial quake.

"It's harder for the rescue teams to get in there to extract people," he said. "Buildings that may have sustained some damage but remain functional can then get another insult and they can collapse."

Syria, already grappling with a years-long humanitarian crisis, major economic woes and a cholera outbreak, was in a "perfect storm" in the wake of the deadly earthquake, according to Brennan.

"The convergence of all these crises is leading to enormous suffering," he said.

(Reporting by Emma Farge and Cécile Mantovani; Writing by Gabrielle Tétrault-Farber; Editing by Bernadette Baum)