AKCAKALE, Turkey - Turkish artillery hit targets near Syria's Tel Abyad border town for a second day on Thursday, killing several Syrian soldiers according to activists and security sources, after a mortar bomb fired from the area killed five Turkish civilians.

Turkey's government said "aggressive action" against its territory by Syria's military had become a serious threat to its national security and sought parliamentary approval for the deployment of Turkish troops beyond its borders.

"Turkey has no interest in a war with Syria. But Turkey is capable of protecting its borders and will retaliate when necessary," Ibrahim Kalin, a senior adviser to Turkish Prime Minister Tayyip Erdogan, said on his Twitter account.

"Political, diplomatic initiatives will continue," he said.

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 neighborhood of the southern border town of Akcakale on Wednesday.

The Syrian Observatory for Human Rights said several Syrian soldiers were killed in the Turkish bombardment of a military post near the Syrian town of Tel Abyad, a few miles across the frontier from Akcakale. It did not say how many soldiers died.

"We know that they have suffered losses," a Turkish security source told Reuters, without giving further details.

NATO said it stood by member-nation Turkey and urged Syria to put an end to "flagrant violations of international law."

The US-led Western military alliance held an urgent late night meeting in Brussels to discuss the matter and in New York, Turkey 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, Turkish UN Ambassador Ertugrul Apakan called the firing of the mortar bomb "a breach of international peace and security."

UN diplomats said Security Council members hoped it would issue a non-binding statement on Thursday that would condemn the mortar attack "in the strongest terms" and demand an end to violations of Turkey's territorial sovereignty.

Members had hoped to issue the statement on Wednesday, but Russia - a staunch ally of Syria's, which along with China has vetoed three UN resolutions condemning President Bashar Assad's government - asked for a delay, diplomats said.

'Aggressive action'

Turkey's parliament had already been due to vote on Thursday on extending a five-year-old authorization for foreign military operations, an agreement originally intended to allow strikes on Kurdish militant 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.

It was not clear who fired the mortar into Turkey, but security sources said it had come from near Tel 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. Information Minister Omran Zoabi conveyed his condolences to the Turkish people, saying his country respected the sovereignty of neighboring countries.

Residents flee, Ankara turns to NATO

Some residents of Akcakale abandoned their homes close to the border and spent the night on the streets. Others gathered outside the local mayor's office, afraid to return to their homes as the dull thud of distant artillery fire rumbled across the town.

"We haven't been able to sleep in our own homes for 15 days, we had to sleep in our relatives' houses further away from the border because it's not safe down there," said shopkeeper Hadi Celik, 42, a father of five.

Washington sees Turkey as a pivotal player in backing Syria's opposition and planning for the post-Assad era. The White House said on Wednesday it stood by "our Turkish ally." But Ankara has found itself increasingly isolated and frustrated by a lack of international consensus on how to end the conflict.

Erdogan long cultivated good relations with Assad, but became a harsh critic after Syria's popular revolt began last year, accusing him of creating a "terrorist state." Erdogan has allowed Syrian rebels to organise on Turkish soil and pushed for a foreign-protected safe zone inside Syria.

On Monday, Syria's foreign minister accused Turkey, the United States, France, Saudi Arabia and Qatar of arming and funding rebels intent on toppling Assad, a charge Ankara has repeatedly denied.

Turkey's military response contrasted with its relative restraint when Syria shot down a Turkish reconnaissance jet in June. Ankara then increased its military presence along its 900-km (560-mile) border with Syria and called a meeting of NATO's North Atlantic Council.

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.

The same article was invoked for the meeting of NATO ambassadors in Brussels late on Wednesday, after which they said the shelling "constitutes a cause of greatest concern for, and is strongly condemned by, all allies".

Some 30,000 people have been killed across Syria, activists say, in a conflict with growing sectarian overtones which threatens to draw in regional Sunni Muslim and Shi'ite powers.

Violence inside Syria intensified on Wednesday with three suicide car bombs and a mortar barrage ripping through a government-controlled district of Aleppo housing a military officers' club, killing 48 people, according to activists.

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