Syrian rebels shot down a Syrian military helicopter Friday near the capital Damascus, the London-based Syrian Observatory for Human Rights said.

The helicopter was downed in Sibka town in the eastern Ghotta region. It was not clear whether the four officers on board were killed or taken hostages, the opposition group said.

Meanwhile, Turkish troops fired at Syria for the second straight day, responding to another mortar shell from Syria that struck Turkish territory, state-run media said. The shelling came hours after Turkish 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 meters 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 in the area immediately responded with fire, 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 Josh 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."

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.

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.