ANKARA, Turkey Turkish troops fired at Syria again Friday, responding to another mortar shell from Syria that struck Turkish territory, the country's state-run news agency said.
The shelling came hours after Prime Minister Recep Tayyip Erdogan called on Syria not to test Turkey's patience.
The Anadolu Agency quoted Gov. Celalettin Lekesiz as saying a mortar shell hit 50 yards inside the border in a rural area near the village of Asagipulluyaz in Hatay province.
No one was hurt by the mortar, but Turkish troops based in the area immediately responded with fire, he said.
Turkish artillery has fired at Syrian targets for two straight days after shelling from Syria killed five Turkish civilians on Wednesday.
Turkey's parliament voted Thursday to allow cross-border military operations in Syria, sharply escalating tensions between the two former allies.
Erdogan on Friday renewed a call for Syria not to challenge Turkey.
"I call on those who try to test Turkey's limits and determination: That would be a grave mistake. We are not bluffing," he said.
The U.S. expressed support for Turkey's response.
"The United States condemns the violence and the aggressive actions of the Syrians," White House spokesman Earnest told reporters traveling with President Barack Obama to Cleveland.
"The Turks have taken some actions that are designed to ensure that their sovereignty is no longer violated by Syrian aggression and we stand shoulder-to-shoulder with them as they take those actions. They are certainly appropriate."
The Syrian uprising against President Bashar Assad erupted in March 2011 and has gradually morphed into a bloody civil war, killing more than 30,000 people, according to activists.
Syrian warplanes and artillery pounded the central city of Homs Friday, subjecting the rebel stronghold to its heaviest bombardment in months, activists said.
The reported tank and mortar shelling as well as airstrikes come alongside a push by government force on another front, the embattled northern city of Aleppo.
The stepped-up pace of government attacks on Syrian cities suggests that the Damascus regime's forces have not been distracted by escalating tensions with its northern neighbor, Turkey.
Also Friday, amateur video posted by activists showed what appeared to be a Syrian government helicopter hurtling to the ground with a trail of white smoke behind it. The Britain-based Syrian Observatory for Human Rights, which has a network of local activists, said it was told by rebels fighters that they shot down the helicopter over Saqba, a town east of Damascus.
Syria imposes tight restrictions on foreign journalists, and the video and rebel claims could not be confirmed independently.
In another development, an activist said rebels captured an air defense base in the Eastern Ghouta area near Damascus earlier this week.
A video showed dozens of gunmen outside a building where smoke is billowing. One of the gunmen says that a "missile air defense battalion" was captured. Another clip showed missiles inside a room.
Activist Mohammed Saeed, who is based in the Damascus suburb of Douma, said rebels captured the base on Monday, but the videos were released late Thursday, three days after the operation. They gave no reason for the delay.
The rebels did not give any other evidence that would confirm the seizure of the base, or identify the location of the video. If confirmed, the capture of a stock of working anti-aircraft missiles would be a boost to a lightly-armed force that says it faces frequent attacks by low-flying helicopters and warplanes.