BEIRUT, LEBANON - Rebel fighters and security forces in Syria clashed near the border with Lebanon and fought over a military barracks in Aleppo on Sunday, while Turkish artillery fired into Syria for a fifth consecutive day in retaliation for cross-border shelling.

On Sunday morning, Syrian forces shelled Tal Abyad, a Syrian border town where rebels recently seized control, according to anti-government activists. Video images posted online purported to show women and children fleeing the area.

Around the same time, a Syrian shell landed about 200 yards across the border from Tal Abyad, near the town of Akcakale in Turkey, the Associated Press reported. Akcakale is the town where a Syrian mortar shell killed Turkish civilians Wednesday, prompting the Turkish government to announce a policy of retaliation for every shell that strays across the border. Turkish forces fired eight shells back into Syria on Sunday. The mounting tensions at the border have raised international concerns that the 18-month-old internal conflict in Syria could draw in neighboring countries or even the NATO alliance, to which Turkey belongs.

It was not clear who fired the shell that landed in Turkey on Sunday, or why. It is not uncommon for mortar and artillery shells to miss their intended targets by significant margins.

Later Sunday, anti-government activists reported clashes between rebels and security forces near an artillery position in Tal Abyad.

In Aleppo, the anti-government Tawhid Brigade said its fighters had penetrated the Hanano military barracks Sunday and were fighting government forces inside the compound. A spokesman for the rebel unit, who uses the pseudonym Abu Mohammed, said that in the fight for the barracks, fighters from his unit were joined by members of Jabhet al-Nusra, an insurgent group said to have ties to Al-Qaida.

Activists reported fierce shelling near the Aleppo citadel, which dates to the 12th century, and said that about 20 shells fell on the al-Sakur neighborhood of the city in a span of a few minutes.

Fighting was also reported Sunday around Damascus, in Homs and in Daraa province in the south, bordering Jordan.