The earthquake hit the town of Petrinja in the central Sisak region at 1319GMT, some 51 kilometers (31.7 miles) southeast of the capital, at a depth of 10 km (6.2 mi.), according to the European-Mediterranean Seismological Centre.

The Interior Ministry confirmed that six people had died so far due to the quake.

"Although the search for the ruins is still ongoing, we are sad to report information about the dead: a girl in Petrinja and five men in Majske Poljane.

"At least 6 people were seriously injured and 20 lightly. Our thoughts are with the families of the victims and injured," the ministry noted. 

Speaking to the Croatian media, Glina Mayor Stjepan Kostanjevic said the bodies of four people were found in the towns near Petrinja.

Kostanjevic added that search and rescue efforts are continuing.

Earlier, Darinko Dumbovic, the mayor of Petrinja, said a 12-year-old girl was killed in the earthquake, and many people were injured.

"This is a horror. This is a tragedy," President Zoran Milanovic said for the earthquake.

He assured that the city will be rebuilt with the EU financial aid, adding: "This city will be rebuilt, but emotionally no one will be able to compensate people."

Prime Minister Andrej Plenkovic arrived at the scene shortly after the earthquake.

"Fortunately, there are barracks in Petrinja, the army is here. Emergency services, police, civil protection, and firefighters are coming from Zagreb, HGSS [Croatian Mountain Rescue Service] will come from Split, military helicopters have flown to pick them up," he said.

"We will have to find alternative accommodation. Containers will come, we will have to remove some people from Petrinja, because it is not safe to be here," he added.

Meanwhile, Serbian President Aleksandar Vucic said they are ready to help Croatia.

According to local media, at least 20 people were hospitalized following the earthquake which caused widespread damage, bringing down roofs and harming building facades, and even making entire buildings collapse.

Earlier, Dumbovic called for help from firefighters from across Croatia.

The government said aid teams across the country were directed to Petrinja.

The quake also caused panic and traffic jams in Zagreb after thousands tried to flee the city and telephone lines were cut.

Neighboring Slovenia has shut down Krsko Nuclear Power Plant as a precaution.

The quake was also felt in some parts of neighboring Bosnia and Herzegovina as well as Serbia.

Turkey conveyed its condolences to Croatia over the earthquake.

In a phone call, Turkish President Recep Tayyip Erdogan sympathized with his Croatian counterpart Zoran Milanovic and said Turkey is ready to provide search and rescue teams and all the help that may be needed.

Separately, the Turkish Foreign Ministry said: "We share the pain of our Croatian friends and sending our condolences to the people of Croatia and to those who lost their beloved ones during the earthquake."

Turkey is ready to provide all kinds of support for the people of Croatia, the statement added.

On Monday, two earthquakes with magnitudes 5.2 and 4.9 rocked the capital and several central towns in Croatia.

Several buildings in the cities of Petrinja and Sisak were damaged by the quakes.

In March, a magnitude 5.3 earthquake hit Zagreb, killing one person and injuring 27 others.

* Iclal Turan contributed to this report from Ankara