Turkey's military continued to launch artillery strikes early on Thursday targeting the Tel Abyad district around 10 km inside the Syrian border, Turkish security sources said.
The sources said a mortar bomb fired from Syria that killed five Turkish civilians on Wednesday had been fired from Tel Abyad. They said soldiers loyal to Syrian President Bashar Assad had been killed by the Turkish artillery fire.
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 Wednesday 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."
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.
"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," Turkish 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 sends condolences to Turkish victims, urges restraint
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.
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.
US 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.