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.
Homs has been one of the flashpoints of the 18-month old uprising against Assad's regime. The focus of fighting has shifted to other areas in recent months, including Aleppo, since a government offensive against rebel strongholds in Homs slowed down in April.
The Observatory said Friday's attack is the worst Homs has seen in five months. The Observatory, which relies on a network of activists on the ground, said the bombardment had hit 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 via Skype on Friday. "An average of five rockets a minute are falling." Abu Rami was speaking from the central rebel-held old quarter known as Old Homs. He asked to be identified by his nickname for fear of regime reprisal.
He said the government forces are 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 have been shelling villages around Homs and the rebel-held town of Rastan, just north of the city.
He said there were no immediate reports of casualties, adding that most residents who still live in rebel-held areas around the city are hiding in shelters.
Activists say most government forces near Homs are stationed outside the town a common pattern in rebel strongholds.
Homs is Syria's third largest city. Regime forces pounded parts of Homs for months, leaving large swaths of the city in ruins by April. Since then level of violence has dropped, although gun battles still frequently break out.
The uprising against Assad erupted in March 2011 and gradually morphed into a bloody civil war, killing more than 30,000 people, according to activists.
The Observatory also said Syrian military has been shelling the neighborhood of Sakhour in Aleppo as government forces battle rebels in the country's largest city.
State-run Syrian TV said that government forces "cleansed Sakhour of terrorists and mercenaries."
Syria's government has always blamed the uprising on what it calls foreign terrorists, despite months of peaceful protests that turned violent after repeated attacks by security forces. The transformation of the conflict into an open war has given an opportunity to foreign fighters and extremists, analysts say.
State TV said troops foiled an attempt by rebels to detonate a car bomb near the Red Crescent Hospital in the Aziziyah neighborhood of Aleppo, added that experts are dismantling the explosives.
Also Friday, Turkey deployed troops along the border with Syria, the day after Turkish parliament approved a bill that authorizes the military to conduct operations outside Turkish border, including Syria. An AP reporter saw Turkish soldiers patrolling the area on foot and in armored personnel vehicles.
People in the border town of Akcakale, where the Syrian shell landed on Wednesday, feared more violence in the volatile border area.
Halil Deniz, who lives in Oncul, says he still fears for his life. Oncul is a village on the Turkey-Syria border to which people fled from Akcakale, where five civilians died when a Syrian shell hit it.
"We do not know if we will live or die," Deniz said. "Children go to other villages in the evenings, and return back when the next day dawns."
"Our store owners, our citizens and our children are all very concerned. We did not sleep until morning," said Ahmet Sabritur, who lives in Akcakale. He praised Turkey's military response to the Syrian shelling, saying that Damascus should be made aware that deaths of Turkish civilians will not go unpunished.
"If our government had not retaliated, maybe it could have been worse," Sabritur said. "They (the Syrians) could have attacked this town a second time."
Turkey's artillery pounded targets inside Syria for two days in retaliation for the killings.