Hundreds have died and thousands are wounded in north-western Syria after Monday's major quake struck a region already devastated by years of civil war.
One desperate relative spoke to dpa from the city of Harem in the hard-hit province of Idlib.
"I received news that my cousin was missing in the earthquake so we came to look for him and help the rescue workers to look for him," Ahmad al-Tawel from the Idlib village of Marata told dpa.
"There are more than 150 families under the rubble. The civil defence rescue teams have done all they can. My friends and I have now volunteered to help and assist the rescue workers. The situation is very catastrophic - really catastrophic," he said.
A volunteer with the White Helmets spoke of the difficulties on the ground. "The work has been continuing for more than 20 hours despite the difficulties due to repeated aftershocks and the sub-zero temperatures," Ibrahim al-Ajam told dpa.
"So many buildings are destroyed.
"The rescue teams are trying with all their means they have to pull out the casualties from under the rubble.
"The work is continuing until this minute, but we call on all organizations to speed up their efforts and to go on high alert to help save the people who are still stuck under the rubble," al-Ajam said.
The White Helmets operate in opposition-controlled parts of Syria. Their work is hampered both by the civil conflict as government forces have virtually blockaded the area, and by sanctions on Syria as a whole.
The Syrian American Medical Society (SAMS) quoted Dr Adel Daghim, director of Idlib Central Hospital, as saying: "What is devastating is the large numbers of families trapped under the rubble that we cannot assist."
"For us in Syria, the situation in hospitals concerning the provision of medical services is under control, but the urgent need now is for rescue teams, because one minute may save a life under the rubble."
SAMS is a non-profit, non-political, professional organization representing thousands of US-Syrian medical professionals in the United States and provides humanitarian assistance to people in Syria's opposition areas.
A senior official for the Aleppo province, Abdul Qadir Dawalibi, described the situation in the city of Aleppo as difficult due to the collapse of a number of buildings in several neighbourhoods and the fear that other buildings might collapse in the aftershocks.
Dawalibi said some 50 buildings have collapsed. "The death toll is expected to rise as rescue teams continue to remove the rubble."
Muhammad Ezz, from Aleppo's al-Kallasa neighbourhood, said that he and his son survived the collapse of a building, but he lost his wife, daughters and mother as they did not wish to leave the building, which collapsed completely.
Ezz told dpa while in Aleppo's university hospital after he suffered minor injuries, "Most of the residents of the building in which we lived were killed because they were unable to get out."
Rescue teams were not able to find many survivors, he added, noting two children who had been found.
Ahmed Hanoura from Aleppo's al-Firdous neighbourhood said that many families were killed because they were sleeping when their buildings collapsed.
Hanoura, 52, said Monday was the hardest day of his life, "I witnessed all the days of the war in the city of Aleppo, and we were hiding from the bombing, but yesterday the fear was greater. We were afraid all the buildings would collapse due to the earthquake."
