Turkish troops, meanwhile, 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 (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 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. 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. 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. The rebels did not give any other evidence that would confirm the seizure of the base, or identify the location of the video.