ACKAKALE, TURKEY - 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 here. Two women and three children were killed leading to resident protests against government officials.

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

The U.S.-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 U.N. Security Council to take the "necessary action" to stop Syrian aggression.

In a letter to the president of the 15-nation Security Council, Turkish U.N. 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." 

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 voted to extend a five-year-old authorization for its military to carry out cross-border operations in Syria an agreement originally intended to allow strikes on Kurdish militant bases in northern Iraq. 

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 ending the conflict. 

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.

U.N. Secretary-General Ban Ki-moon had urged Turkey to keep all channels of communication open with Syria and 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". A rare emergency NATO meeting was also called. 

On Thursday, Syrian officials admitted responsibility for the attacks and told the UN such an incident would not happen again. But Turkish artillery shelled several Syrian military targets on Thursday, leading to fresh concern that the conflict could continue to spread.

- Reuters, TAAN