Turkey's Parliament authorized military operations against Syria on Thursday and its military fired on targets there for a second day after deadly shelling from Syria killed five civilians in a Turkish border town.
For its part, Syria admitted it was responsible for the shelling that killed five people in Turkey and formally apologized for the deaths, a top Turkish official said.
Deputy Prime Minister Besir Atalay said Syria has reassured the U.N. that "such an incident will not occur again."
The border violence has added a dangerous new dimension to Syria's civil war, dragging Syria's neighbors deeper into a conflict that activists say has already killed 30,000 people since an uprising against President Bashar Assad's regime began in March 2011.
Atalay said Parliament's authorization was not declaration of war on Syria but gives Turkey the right to respond to any future attacks from Syria.
"The bill is not for war," Atalay said. "It has deterrent qualities."
Cross-border tensions escalated Wednesday after a shell fired from inside Syria landed on a home in the Turkish village of Akcakale, killing two women and three of their daughters and wounding at least 10 others, according to Turkish media.
The bill Thursday opens the way for unilateral action by Turkey's armed forces inside Syria without the involvement of Turkey's Western or Arab allies. Turkey has used a similar provision to repeatedly attack suspected Kurdish rebel positions in northern Iraq.
Still, Atalay said Turkey's "main priority" was to "act together with the international community."
"That is why we called on NATO and the United Nations to take up the issue," Atalay said.
The NATO military alliance, of which Turkey is a member, met at an emergency session in Brussels and condemned the attack on Turkey. NATO demanded "the immediate cessation of such aggressive acts against an ally" and urged the Syrian regime to "put an end to flagrant violations of international law."
The Turkish response to the shelling was prompt -- it fired salvos of artillery rounds deep inside Syria.
Mustafa Guclu, a witness in Akcakale, said the Turkish military fired five rounds of artillery "after midnight" toward Syria and another round around 5 a.m. on Thursday.
"I have not heard any more shelling since then," he told the Associated Press.
The Syrian mortar shell damaged the door and walls of a house in Akcakale, while shrapnel poked holes and shattered windows of neighboring houses and shops.
Some residents of Akcakale abandoned their homes close to the border and spent the night on the streets. Others gathered outside the mayor's office, afraid to return to their homes as the dull thud of distant artillery fire rumbled.
Turks have grown weary of the burden of involvement in the Syrian conflict, which includes the hosting of 90,000 Syrian refugees in camps along the border.
Yet Turkey is still loath to go it alone in Syria, and is anxious for any intervention to have the legitimacy conferred by a U.N. resolution or the involvement of a broad group of allies. Turkey is mindful in part of inconclusive ground missions, mostly in the 1990s, against Kurdish guerrillas based in northern Iraq, as well as the bitter lessons of being seen as an occupying power that are associated with the U.S.-led invasion in Iraq.
Reaching deeper into history, Turkey is aware of Mideast sensibilities over Ottoman rule over much of the region.
On a visit to Pakistan on Thursday, Russian Foreign Minister Sergey Lavrov expressed his government's concern over the escalation of tensions.
Lavrov, speaking at a press conference in Islamabad, said Syria has assured Russia, an ally, that such an incident as the shelling that killed the Turks will not happen again.
"It is of great concern for us," Lavrov said. "This situation is deteriorating with every coming day."
Germany's foreign minister, Guido Westerwelle, said NATO members agree on the need for solidarity but also on prudence in reacting to events on the Turkish-Syrian border.
The Turkish retaliatory shelling and steps to authorize possible military intervention against Syria were the latest events to sharply escalate tensions between the two former allies.
In June, Turkey reinforced its border with anti-aircraft missiles and threatened to target any approaching Syrian military elements after Syrian forces brought down a Turkish jet, killing its two pilots. Turkey said the plane was in international airspace, countering Syrian claims that it was in Syrian airspace.