Turkish artillery hit targets inside Syria on Wednesday after a mortar bomb fired from Syrian territory killed five Turkish civilians, while NATO called for an immediate end to Syria's 'aggressive acts'.

In the most serious cross-border escalation of the 18-month uprising in Syria, Turkey hit back at what it called 'the last straw' when a mortar hit a residential neighborhood of the southern border town of Akcakale.

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 later on Tuesday 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 flagrant violation of international law as well as a breach of international peace and security'.

There were no immediate details of the Turkish strikes against Syria, nor was it clear who had fired the mortar into Turkish territory, but security sources said 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," Prime Minister Tayyip Erdogan's office said in a statement.

"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.

Turkey's parliament was due to vote on Thursday on extending a five-year-old authorization for its military to carry out cross-border operations, an agreement originally intended to allow strikes on Kurdish militant bases in northern Iraq.

That vote would now be extended to include operations in Syria, a ruling party deputy told Turkish television.

Residents of Akcakale 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 who was among the crowd outside the mayor's office.

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 Syrian President Bashar al-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 organize 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.

NEXT STEPS

Syria's bloodshed has crept ever closer to Turkey's border as the uprising against Assad slides into civil war.

"Over the last 20 to 25 days there have been very heavy clashes on the Syrian side. We have felt the effects of these in Akcakale," Turkish Labour Minister Faruk Celik, an MP for the province where Akcakale is located, told parliament.

Residents, infuriated by the increasing spillover of violence from Syria, took to the streets shouting protests against the local authorities after the mortar bomb struck in a residential area, killing two women and three children.

"People here are anxious, because we got hit before," Ahmet Emin Meshurgul, local head of the Turkish Red Crescent, told agencies.

"The security forces tried to convince people to empty the neighborhood near the border, but now we've been hit right in the middle of the town."

UN Secretary-General Ban Ki-moon had urged Turkey to keep all channels of communication open with Syria. He later issued a statement calling on "the Syria government to respect fully the territorial integrity of its neighbors as well as to end the violence against the Syrian people."

U.S. Secretary of State Hillary Clinton expressed outrage at the mortar from Syria and said Washington would discuss with Ankara what the next steps should be, calling the spread of violence a 'very, very dangerous situation'.

Turkish Foreign Minister Ahmet Davutoglu said Clinton had assured him of Washington's full support at the United Nations and NATO.

NATO MEETING

Turkey's military response contrasted with its relative restraint when Syria shot down a Turkish reconnaissance jet in June. Ankara 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'.

"The alliance continues to stand by Turkey and demands the immediate cessation of such aggressive acts against an ally, and urges the Syrian regime to put an end to flagrant violations of international law," a statement released after the meeting added.

It said recent aggressive acts by Syria were a 'clear and present danger to the security of one of (NATO's) allies'.

Turkish Deputy Prime Minister Bulent Arinc said after the mortar attack: "This latest incident is the last straw. Turkey is a sovereign country. Its own soil has been attacked."

"There must be a response to this under international law," he said, according to Turkey's Cihan news agency.

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.

Efforts to address the conflict at the United Nations have been blocked by a standoff in the Security Council between Western powers seeking a tough stance against Assad and Russia and China, which fear a UN resolution against Syria would be the first step towards military intervention.

An Egyptian attempt to bring together Egypt, Turkey, Iran and Saudi Arabia to search for a regional solution to the crisis also appeared to be going nowhere after Saudi Arabia stayed away for a second time from a meeting of the four countries.