Anti-government forces in Syria say they have advanced into one of the major cities in the central part of the country, amid continuing fighting with government troops.

The rebels claimed on social media on Thursday that they had entered the city of Hama and taken control of a prison.

The government forces said in a statement that they acknowledged the rebels' movements. They said their units were redeployed outside the city to protect civilian lives and prevent urban combat.

Hama is a key transportation hub located between the major northern city of Aleppo and the capital, Damascus.

The rebels captured much of Aleppo as they began stepping up their offensive last week, and had since pushed toward Hama.

Syria's military struck back by carrying out air raids jointly with Russian forces. Moscow backs the Syrian government led by President Bashar al-Assad.

Reuters news agency reported that the rebels' capture of Hama will send shockwaves through the Assad administration as the city had remained under government control since the civil war erupted in 2011.

Reuters also said fears will grow about the rebels marching south toward the capital.

The Syrian Observatory for Human Rights said more than 700 people, including civilians, had died in the fighting.

UN Secretary-General Antonio Guterres told reporters on Thursday that he spoke with Turkish President Recep Tayyip Erdogan, and stressed the need for immediate humanitarian access and a return to the UN-facilitated political process to end the conflict.

The UN chief said, "Tens of thousands of civilians are at risk in a region already on fire." He said the current crisis resulted from a "chronic collective failure of previous de-escalation arrangements."

Guterres said that, after 14 years of conflict, it is time for all parties to hold serious dialogue with UN special envoys. He said a new comprehensive approach should be drawn up in line with a UN Security Council resolution to resolve the crisis.