BEIRUT — The Turkish prime minister announced Wednesday night that Turkey had fired artillery at targets in Syria, in retaliation for Syrian mortar fire that fell in a Turkish border town and killed five Turkish civilians.

It was the first instance of significant fighting on the Turkish-Syrian border since the unrest began in Syria last year, and raised the prospect of greater involvement by the NATO alliance, to which Turkey belongs.

"This atrocious attack was immediately responded to adequately by our armed forces in the border region, in accordance with rules of engagement,"a written statement from the office of the prime minister, carried by the semiofficial Anatolian News Agency, said."Targets were shelled in locations identified by radar."

"Turkey, in accordance with the rules of engagement and international law, will never leave such provocations by the Syrian regime against our national security unrequited," the statement added.

NATO said it would convene an urgent meeting on the issue Wednesday. Before firing into Syria, Turkey contacted the United Nations and NATO to protest the killings and express its "deepest concern."

Secretary of State Hillary Rodham Clinton said she was "outraged" by the mortar attack in Turkey.

The five Turkish civilians — a woman, her three children and a relative — were killed in the town of Akcakale, and their deaths were the first reported deaths caused by the stray shells that have frequently flown across the border, a Turkish official said.

It was unknown whether the mortar shells were fired by Syrian government forces or rebels fighting to topple the government of President Bashar Assad. The Turkish response seemed to assume the Syrian government was responsible.

In Aleppo, Syria, on Wednesday, several huge explosions struck a government-held district, shearing off the fronts of two tall buildings, killing dozens of people and filling the streets with rubble in a square near a public park, according to video, photographs and reports from the Syrian government and its opponents.

The government blamed its opponents and said civilians were among the dead. The Syrian Observatory for Human Rights, which is based in Britain and relies on a network of observers inside Syria, said most of the dead were from the security forces and the explosions went off after clashes between gunmen and guards.

Citing medical sources, the Observatory said 40 people were killed and 90 wounded. The Syrian government said at least 34 people died.