ISTANBUL — Turkey said Wednesday that it shelled targets within Syria as retaliation for a mortar shell that landed across the Turkish border and killed five civilians, a move that increases the risk of escalating the bloody civil war into a regional conflict and ratchets up pressure for further international involvement.

The North Atlantic Treaty Organization, of which Turkey is a member and whose charter calls in some cases for collective action when one of its members is targeted militarily, called an emergency meeting on Wednesday evening to discuss the crisis, as Turkey’s civilian and military leaders said Parliament would consider a motion on Thursday to permit further military action within Syria.

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

And while suicide bombers killed dozens on Wednesday as violence surged in Syria’s largest city, Aleppo, it was the cross-border strike that raised the stakes in a civil war that has left tens of thousands dead and forced more than a million people from their homes. The war has defied exhaustive diplomatic efforts by the global community. The events may increase pressure for the West to take military action, something Turkey has supported. The United States and its allies have balked at engaging in another armed conflict in the Muslim world that would be far riskier than NATO’s intervention in Libya, which helped oust Col. Muammar el-Qaddafi.