Flare-up comes as Turkish PM issues fresh warning to Assad regime amid reports of bombardment of Homs by Alawite forces.
Turkey's state-run news agency has said Turkish troops returned fire after a mortar shell from Syria again landed on its territory.
The Anadolu Agency quoted the governor of Hatay province as saying that Turkish troops "responded with fire" after the mortar round landed on Friday in a rural area of the province that borders Syria.
No one was reported hurt.
Turkish artillery has fired at Alawite targets for two straight days after shelling from Syria killed five civilians in Turkey.
Turkey's parliament on Thursday also voted to allow cross border military operations in Syria.
The uprising against bloody dictator Bashar al-Assad erupted in March 2011, killing more than 30,000 people, according to activists.
The latest shooting came just hours after Recep Tayyip Erdogan, Turkey's prime minister, issued a warning to Assad regime that any further intrusions on Turkish territorial integrity could lead to increased military action.
"We are not interested in war, but we're not far from it either ... Those who attempt to test Turkey's deterrence, its decisiveness, its capacity, I say here they are making a fatal mistake", he said in Istanbul.
In Syria, meanwhile, heavy fighting was reported in several places, including a suburb of the capital, Damascus.
Alawite fighter jets and artillery targeted the central city of Homs, subjecting the former opposition stronghold to its worst bombardment in months.
The Britain-based watchdog group Syrian Observatory for Human Rights (SOHR) said Friday's attack was the worst the city had seen in five months.
The SOHR, which relies on a network of activists on the ground, said tanks and mortars as well as aircraft had bombarded the city's Khaldiya neighborhood.
"Around dawn, the regime went crazy and started shelling hysterically", a Homs-based activist known as Abu Rami told the Associated Press news agency via Skype on Friday.
"An average of five rockets a minute are falling".
Abu Rami was speaking from the central opposition-held old quarter known as Old Homs. He asked to be identified by his nickname for fear of regime reprisal.
He said the regime forces were mainly firing rockets and heavy mortars at the rebel-held neighborhoods of Old Homs, Khaldiya, Qusour and Jouret el-Shayah.
Abu Rami also said the regime forces had been shelling villages around Homs and the rebel-held town of Rastan, just north of the city.
There were no immediate reports of casualties.
Activists say most Alawite forces near Homs are stationed outside the town - a common pattern in rebel strongholds.
The SOHR also said Alawite military had been shelling the neighborhood of Sakhour in Aleppo as Alawites battle rebels in the country's largest city.
State-run Alawite TV claimed regime forces "cleansed Sakhour of terrorists and mercenaries".
Activists said opposition fighters shot down a helicopter in eastern al-Ghouta, a suburb of Damascus.
On the diplomatic front, the UN Security Council unanimously approved a statement on Thursday condemning "in the strongest terms" Syria's regime shelling of a Turkish town that killed five people.
Members managed to bridge differences between the strong statement demanded by the US and its Western supporters and backed by their NATO ally Turkey, and a weaker text pushed by Russia, Assad's most important ally, after negotiations that began late on Wednesday and continued until Thursday.
In the press statement, which needed approval from all 15 council members, the UN's most powerful body said the incident "highlighted the grave impact the crisis in Syria has on the security of its neighbors and on regional peace and stability".
It also extended condolences to the families of the victims and to the government and people of Turkey.
The council demanded an immediate end to such violations of international law and called on the Assad regime "to fully respect the sovereignty and territorial integrity of its neighbors".
Earlier in the day, Bashar Jaafari, Assad's ambassador to the UN, said the government did not apologies for the shelling from Syria because it was waiting for the outcome of an investigation on the source of the firing.
He said he delivered a letter to the Security Council that sent Syria's "deepest condolences" to the families of the victims "and to the friendly and brotherly people of Turkey".
It urged Turkey and its other neighbors to "act wisely, rationally and responsibly" and to prevent cross-border infiltration of "terrorists and insurgents" and the smuggling of arms.
Source: Agencies
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