Turkey's parliament has authorised cross-border military action against Syria, if deemed necessary by the government.
The mandate, valid for one year, was passed by 320 votes in the 550-seat Turkish parliament, the Anatolia news agency reported on Thursday.
Besir Atalay, Turkey's deputy prime minister, said authorising the use of force in Syria was not a declaration of war but was intended as a deterrent.
The vote came as Turkey resumed shelling Syrian government military positions on Thursday morning in retaliation for a mortar attack which landed over its border in southeastern Turkey killing five of its citizens - a woman and four children from the same family.
"The Syrian side has admitted what it did and apologised," Atalay said.
Turkish state media said that the attacks by artillery units based in the border town of Akcakale were continuing.
The diplomatic developments came as violence continued in Syria itself. Opposition activists reported several large explosions in a Damascus suburb.
A London-based rights monitor reported 63 deaths across the country on Thursday.
Syrian troops injured
Two Syrian troops were injured as a result of overnight Turkish shelling at a base near the Syrian border town of Tal al-Abyad, Bashar al-Jaafari, Syrian ambassador to the UN, said.
Speaking at the UN on Thursday, Jaafari said though their men had been injured in the retaliatory attacks, the forces of Bashar al-Assad, Syrian president, "practised self-restraint" and did not respond.
Jaafari said Syria was "seriously investigating" the source of the artillery fire, but said that in such instances, "states and governments should act wisely, rationally and responsibly".
Recep Tayyip Erdogan, Turkey's prime minister, addressing the shelling in Akcakale for the first time, said though this is not the first instance of Turkey's territorial integrity being challenged by Syrian artillery, Turkey has shown "restraint and patience".
However, Erdogan, speaking in the Turkish capital, said: "Turkey has no intention of starting a war" in the region.
"All we want in this region is peace and security. We have no intention of starting war. We are aware of the outcome, consequences, of war in Iraq and Afghanistan ... we see the same in Syria".
The parliament had already been due to vote on Thursday on extending a five-year-old authorisation for foreign military operations, an agreement originally intended to allow strikes on Kurdish bases in northern Iraq.
But the memorandum signed by Erdogan and sent to parliament overnight said that despite repeated warnings and diplomatic initiatives, the Syrian military had launched aggressive action against Turkish territory, presenting "additional risks".
"This situation has reached a level of creating a serious threat and risks to our national security. At this point the need has emerged to take the necessary measures to act promptly and swiftly against additional risks and threats," it said.
'The last straw'
Al Jazeera's Anita McNaught, reporting from Akcakale, said the addition of the Syrian resolution was a "knee-jerk response" to Wednesday's shelling.
In the most serious cross-border escalation of the 18-month uprising in Syria, Turkey hit back after what it called "the last straw" when a mortar hit a residential neighbourhood of the southern border town of Akcakale on Wednesday.
Al Jazeera's Jacky Rowland reports on the NATO reaction
Security sources said the mortar had come from near Tal al-Abyad and that Turkey was increasing the number of troops along its border.
"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," Erdogan's office said in a statement late on Wednesday.
"Turkey will never leave unanswered such kinds of provocation by the Syrian regime against our national security."
Syria said it was investigating the source of the mortar bomb and urged restraint. Omran Zoabi, information minister, conveyed his condolences to the Turkish people, saying his country respected the sovereignty of neighbouring countries.
Following the attack, Bulent Arinc, the deputy prime minister, said Turkey was "not blinded by rage".
"There is definitely a response to it [the attack] in international law ... We are not blinded by rage, but we will protect our rights to the end in the face of such an attack on our soil that killed our people."
Al Jazeera's Andrew Simmons , reporting from Antakya on the Turkish-Syrian border, said Arinc's mention of "certain responsibilities" contained within NATO treaty articles, could mean that Turkey responded without consulting international bodies first.
NATO said it stood by member-nation Turkey and urged Syria to put an end to "flagrant violations of international law".
Al Jazeera's McNaught said that one has to ask the question of whether Syria "would want to draw Turkey into the conflict, and would Turkey want this to be the start of a larger and widening escalation of the battle regionally".
The US-led Western military alliance held an urgent late night meeting in Brussels to discuss the matter.
In-depth coverage of escalating violence across Syria
That meeting was only the second time in NATO's 63-year history that members had convened under Article 4 of its charter which provides for consultations when a member state feels its territorial integrity, political independence or security is under threat.
Turkey also asked the UN Security Council to take the "necessary action" to stop Syrian "aggression".
In a letter to the president of the 15-nation Security Council, Ertugrul Apakan, Turkey's UN ambassador, called the firing of the mortar bomb "a breach of international peace and security".
Security Council members managed to bridge differences between the strong statement demanded by the United States and its Western supporters and backed by their NATO ally Turkey, and a weaker text pushed by Russia, Syria's most important ally, after negotiations that began late Wednesday and continued through Thursday.
In the press statement, which needed approval from all 15 council members, the UN's most powerful body said the incident "highlighted the grave impact the crisis in Syria has on the security of its neighbours and on regional peace and stability".
It also extended condolences to the families of the victims and to the government and people of Turkey.
The council demanded an immediate end to such violations of international law and called on the Syrian government "to fully respect the sovereignty and territorial integrity of its neighbours".
Russia's agreement that the Syrian shelling violated international law was a key concession by Moscow.