A car bomb has exploded near the police headquarters in the Syrian capital Damascus, killing one officer, the government said.
It blamed Sunday's attack on what it called "terrorists", the term it uses to describe rebels fighting President Bashar al-Assad's regime. The state news agency said the bomb exploded in a vehicle in the car park of the building on Khaled bin al-Walid Avenue in central Damascus.
Witnesses told the AFP news agency the blast was followed by heavy gunfire. This is the latest in a string of bombings against high-profile security installations in the capital.
Earlier, Turkey's military fired numerous artillery rounds into Syria in retaliation after a shell fired from Syria landed in the Turkish border town of Akcakale.
The Syrian shell landed in the grounds of a public building on Sunday and there were no casualties, Abdulhakim Ayhan, the mayor of Akcakale, told reporters.
"Thank God there were no victims. Turkish artillery immediately responded to the shots that came from Syria," Ayhan said hours before explosions hit Damascus.
Turkish media said there was minor damage to the public building, which had been previously evacuated.
It was the second time that Turkey returned fire on Syrian army positions since Syrian shelling killed five civilians in Akcakale on Wednesday.
Al Jazeera's Decrim Gurkan, reporting from Akcakale, said that he heard "six firings" of mortar rounds from Turkish security forces towards Syria, which were followed by a calm.
"At first there was panic but it didnt last too long. The situation is calm here," he said.
"People here on the streets, in the cafes, the parks. We asked them if they were afraid and they said they got used to it," our correspondent added.
"We see there are some people packing up and leaving the city, especially the ones living near the border area."
"Authorities were thinking about opening schools tomorrow, but after the latest incident, many teachers left this town so the opening of schools is postponed."
Recep Tayyip Erdogan, Turkey's prime minister, warned Syria on Friday that Turkey would not shy away from war if provoked, but a series of mortar bombs fired from Syria have continued to hit Turkey since then.
The exchanges of fire are the most serious cross-border violence in Syria's conflict.
Rebels seize northern town
Meanwhile, Syrian rebels have cemented their control of the country’s northern frontier with Turkey, as their bastions in other parts of the country come under heavy shelling.
Opposition fighters reportedly seized the town of Khirbat al-Joz in the northwest province of Idlib after fierce clashes with regime forces.
"The fighting [a day earlier] lasted more than 12 hours and resulted in at least 40 dead among the regular forces, including five officers, and nine [rebel] fighters," the UK-based Syrian Observatory for Human Rights said.
Meanwhile, in the province of Damascus, Syrian state television said that government forces had pushed rebels out of two of their strongholds in Damascus province, Qudsaya and Hameh, where activists said that the bodies of 20 men were found.
Also on Sunday, Syrian troops pressed their offensive to retake rebel-held areas in Homs and southern villages on the border with Jordan.
In Aleppo, where fighting has raged since mid-July, the bombardment targeted the embattled district of Sakhur in the east and Kalasseh in the southwest.
Tension with Turkey
On the border with Turkey, regime forces reportedly pounded the town of Tal-Abyad in the northern province of al-Raqqa.
Nearly 80 per cent of towns and villages along the Turkish border are outside the control of Damascus, according to activists.
In-depth coverage of escalating violence across Syria
AFP news agency correspondents have passed through large swathes of territory in the Idlib and Aleppo provinces of northern Syria that have fallen outside government control, with residents managing their own affairs.
That followed heavy bombardments of Syrian military positions near the border since Wednesday, when a shell smashed into a Turkish town killing five civilians and sparking outrage in Ankara and a UN Security Council condemnation.
Turkey had on Friday shelled a Syrian military position south of Tal- Abyad in retaliation after a Syrian shell landed on its territory near the border.
Amid the growing tension between the Damascus and Ankara, Ahmet Davutoglu, the Turkish foreign minister, said on Saturday that Faruq al-Shaara, Syria's vice-president, "is a man of reason" who could replace President Bashar al-Assad as the head of a transition administration to stop the conflict in the country.
Davutoglu said on the public television channel TRT: "Faruq al-Sharaa is a man of reason and conscience and he has not taken part in the massacres in Syria. Nobody knows the (Syrian) system better than he."
He stressed that the Syrian opposition "is inclined to accept Sharaa" as the future leader of the Syrian administration.
Sharaa, the most visible Sunni Muslim figure in the minority Alawite-led government, is trusted by the regime and was foreign minister for 15 years before becoming vice president in 2006.
Reports that he had defected in August were denied by Damascus, but some opposition leaders say he is apparently under house arrest.
Davutoglu said he was convinced that the Syrian vice president was still in Syria.
Turkey, which shares a border of 900 km with Syria and hosts nearly 100,000 Syrian refugees on its territory, openly supports rebels from the Free Syrian Army and has called for Assad's ouster.