The 7.7 magnitude earthquake that hit Türkiye on Monday was "one of the largest inland earthquakes" ever recorded, an expert said.

"It was an unexpected big earthquake," said professor Shinji Toda, underscoring the intensity of the earthquakes that hit southern Türkiye a day earlier, resulting in the death of more than 5,400 people.

"It was one of the largest inland earthquakes in recorded history," said Toda, who is a professor at the International Research Institute of Disaster Science at Japan-based Tohoku University.

He told the Japanese daily Asahi Shimbun that the earthquake "is thought to have been active over a wide area of about 150 to 200 kilometers (around 90 to 125 miles) west of the East Anatolian Fault Zone, which is known as an active fault."

East Anatolian Fault is a strike-slip fault located on the boundary between the Arabian Plate and the Anatolian Plate, the professor said.

"In addition to the magnitude of the earthquake, the shallowness of the epicenter caused extensive damage," the Japanese professor said in an interview published on Tuesday.

Toda said the "amount of energy" released by the earthquake that hit the southern parts of Türkiye on Monday "is more than 10 times that of the 1995 Great Hanshin-Awaji Earthquake and the 2016 Kumamoto Earthquake."

The 7.3 magnitude Great Hanshin earthquake, or Kobe earthquake, occurred on Jan. 17, 1995, in Japan's southern Hyogo province; while the Kumamoto earthquakes were a series of earthquakes, including a magnitude 7.0 mainshock that struck on April 16, 2016, beneath the Kumamoto City of Kumamoto province in Japan's Kyushu region.

Türkiye has declared a three-month state of emergency in 10 provinces hit by earthquakes.

A 7.7 magnitude tremor early on Monday struck the Pazarcık district of Kahramanmaraş province – the epicenter of the quakes, then about nine hours later, a 7.6 magnitude quake centered in Kahramanmaraş's Elbistan district rocked the region, affecting several other provinces, including Adana, Adıyaman, Diyarbakır, Gaziantep, Hatay, Kilis, Malatya, Osmaniye and Şanlıurfa.

The earthquake also hit northern Syria, killing and injuring thousands, and was felt in Lebanon as well.

"We are facing one of the biggest disasters not only in the history of the Turkish republic but also of our geography and the world," Turkish President Recep Tayyip Erdoğan said.

"Our biggest relief is that over 8,000 of our citizens have been rescued from the rubble so far," Erdoğan said.

Türkiye also issued a Level 4 alarm, which includes a call for international aid.

Earlier, Türkiye's Disaster and Emergency Management Authority (AFAD) had said the magnitude of the first earthquake was 7.4 but later revised it to 7.7.

The AFAD said the quakes were followed by 243 aftershocks.