BEIRUT – The Turkish military struck targets inside Syria on Wednesday in the most serious escalation in international tensions since the Syrian revolt erupted 19 months ago.

A brief statement issued by Prime Minister Recep Tayyip Erdogan’s office said the Turkish army had hit unspecified Syrian targets in retaliation for a shell fired from Syria that killed at least five civilians in a Turkish border town.

“Our armed forces in the border region responded immediately to this abominable attack in line with their rules of engagement; targets were struck through artillery fire against places in Syria identified by radar,” the statement said.

Turkey summoned its NATO allies to an emergency meeting late Wednesday to discuss the attack, and the alliance later issued a statement saying it “continues to stand by Turkey” and urging Syria to cease what it called “such aggressive acts” and “flagrant violations” of international law.

In Washington, the White House also condemned the Syrian shelling and confirmed the United States’ solidarity with Turkey. “We stand with our Turkish ally and are continuing to consult closely on the path forward,” spokesman Tommy Vietor said.

In a statement issued before the Turkish retaliation, U.N. Secretary-General Ban Ki-moon urged restraint, warning that the attack demonstrated the risks the Syrian conflict poses in the region.

“Syria’s conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbors,” he said.

Though Syrian artillery shells fired in the ongoing fight between rebels and government forces have exploded inside Turkey in the past, the attack Wednesday marked the first time Turkish civilians had been killed.

The website of the Turkish daily Today’s Zaman reported that the shell killed a woman and four children in the border town of Akcakale and injured at least 13 other people.

The strike culminated months of rising tensions between Turkey and Syria since Syria shot down a Turkish jet in June, killing two pilots over international waters.

Earlier Wednesday, state media reported that three massive bomb blasts in the heart of Syria’s commercial capital, Aleppo, had killed at least 31 people and caused widespread damage to a major square.

Government forces and rebels have been battling for weeks for control of the city.

The bombs, which apparently targeted a military officers’ club, went off in rapid succession shortly before 8 a.m. local time, authorities said.

The club was demolished, and the blasts also tore the facades off several nearby buildings in Saadallah al-Jabri Square, a part of the city that is tightly controlled by the government.

The official Syrian Arab News Agency attributed the blasts to suicide bombers. It described a major assault in which two suicide bombers detonated cars in the square, after which three suicide bombers approached the scene wearing Syrian army uniforms and explosive vests. The three were killed, it said, without giving further details.

Rebels in the city said all of the casualties were members of the security forces, who maintain a heavy presence in the square.