Despair is mounting in Turkey as the death toll from a pair of major earthquakes in the country and neighboring Syria climbs above 17,000, with survivors and opposition politicians expressing frustration at the speed of the disaster response.

Hope of finding survivors is dimming. Freezing temperatures in the towns and cities flattened by Monday's quakes have lengthened the odds, even as international rescue teams arrive in Turkey with equipment and rescue dogs able to detect the scent of humans beneath piles of rubble.

"The situation is very bad," said Mohammed Farhan Khalid, the leader of a team of Pakistani rescuers in the shattered southeastern city of Adiyaman, who compared the Turkish earthquakes to the 2005 earthquake in Kashmir that killed tens of thousands. "More rescue and relief is required."

The death toll in Turkey - which sustained the majority of the fatalities so far - has risen to at least 14,014, according to the government. The full extent of the disaster may not be clear for weeks, given the scale of the damage, with entire neighborhoods reduced to ruins. Already, it ranks as the world's deadliest earthquake disaster in more than a decade.

At least three U.S. citizens were among those killed in southeastern Turkey, according to the State Department.

In Syria, rescue efforts have been hampered by the aftereffects of a war that has left the country divided into government and rebel-controlled areas. In government-held parts of Syria, state media reported that at least 1,262 people had been killed and 2,285 injured.

Rescuers in the rebel-held northwest region reported early Thursday that more than 1,900 were dead and 2,950 people injured, a tally they expect to rise in coming days as many people remain buried beneath the rubble.

A U.N. convoy of six trucks carrying humanitarian aid was "in the process of crossing" into northwest Syria on Thursday via a corridor through Turkey, said Jens Laerke, spokesman for the U.N. Office for the Coordination of Humanitarian Affairs.

U.N. officials have expressed hope that aid deliveries can resume via the crossing at Bab al-Hawa into the rebel-held region, where millions of people are displaced and in dire need and where many live in camps. "We consider this a test, that things can restart," Laerke said Thursday.

Across the border in Turkey, President Recep Tayyip Erdogan went to densely populated Gaziantep on Thursday, near the epicenter in southern Turkey, where the quake has decimated residential blocks.

Erdogan has urged citizens to be patient, and pledged to rebuild the many shattered towns and cities within a year - a tough ask when the government's latest estimate on the number of collapsed buildings is more than 6,400. He also said that the Turkish government would offer families 10,000 Turkish lira, or around $530.

Washington Post journalists in southern Turkey saw survivors scuffle for tents distributed by aid agencies and scramble for blankets. Families who had missing loved ones sifted through the debris without assistance, with heavy equipment arriving days after the temblors struck.

Access to social media platforms Twitter and TikTok was restricted for some Turkish users on Wednesday. Internet-monitoring group NetBlocks later stated that Twitter services were restored after Turkish policymakers met with Twitter officials.

Ankara has previously cracked down on social media companies in the wake of disasters or periods of political scandal or unrest. Erdogan is facing an election in a few months and recovering from the earthquakes will be a major test of his two-decade grip on power. Even before the earthquakes, the country was grappling with historically high inflation and economic hardship that has dampened his popularity among voters.

Meral Aksener, a right-wing politician who founded the rival Iyi Party, or Good Party, denounced what she characterized as apparent censorship of social media at a time when it was being used by citizens as a vital means of conveying news about earthquake victims.

Vice President Fuat Oktay attributed the outage to technical issues and noted that other social media sites were still available.

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The Washington Post's Amar Nadhir in London, Zeynep Karatas in Adiyaman, Turkey, and Paulina Villegas, Naomi Nix and Anumita Kaur in Washington contributed to this report.