United Nations Secretary-General Ban Ki-moon warned that the development demonstrates "how Syria's conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbors." France called on the U.N. Security Council to condemn the Wednesday strike on Akcakale.

And Russia, a friend of the Syrian government, called for restraint.

"Through our ambassador to Syria, we have spoken to the Syrian authorities who assured us ... that what happened at the border with Turkey was a tragic accident, and that it will not happen again," said Foreign Minister Sergey Lavrov, according to state-run media. "We think it is of fundamental importance for Damascus to state that officially."

Turkey said Syrian forces conducted the shelling. The attack struck a chord across the nation.

Asli Aydintasbas, a columnist for the Turkish newspaper Milliyet, lobbied for a forceful Turkish reaction in her newspaper column Thursday.

"In my view, Turkey has no other option but to retaliate," she told CNN in a telephone interview.

"Syrians were testing our resolve persistently. This was an effort to signal to Turkey that it needs to stay out of the Syrian crisis. Not only the safety of our citizens, but also our prestige in the region was at stake. In this neighborhood, countries have to have deterrent capabilities in order to survive. Turkey has to respond in a fashion to show that it is not a country to be messed with."

Akcakale has been rocked by previous fighting just across the border in Syria. Last month, Turkish residents watched as Syrian shells crashed into Syrian territory, barely a stone's throw away from the Turkish border fence. The close artillery barrage forced Turkish authorities to temporarily shut schools in Akcakale and close off roads leading to the Syrian border.

Turkey has had a key role in calling for a transition of power in Syria, hosting international diplomats at ad hoc meetings of the Friends of Syria, a group that was formed after the U.N. Security Council failed to take action.

Last March, Turkey shuttered its embassy in Damascus and the Syrian government declared Turkey's ambassador, Omer Onhon, persona non grata.

Erdogan has repeatedly denounced al-Assad, publicly calling on him to step down after accusing him of massacring his own people. The Syrian government, meanwhile, has accused Turkey of arming and funding Syrian rebels.

CNN journalists have witnessed light weapons in the form of assault rifles, rocket-propelled grenades and machine guns coming from Turkey to Syrian rebels.

In addition, Turkey is currently hosting more than 93,000 Syrian refugees in camps. Turkish officials estimate an additional 40,000 to 50,000 unofficial refugees live in the country outside refugee camps.

On Wednesday, the North Atlantic Council, NATO's most senior political governing body, said it stands by Turkey, which is a member of the treaty organization. The alliance "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," the council said after an emergency meeting. The Syrian government has a recent pattern of "aggressive attacks" at NATO's southeastern border, in Turkey, it said.

In other developments:

Battle for Damascus

Widespread fighting was reported Thursday across Syria, with at least 21 Republican Guards killed in an explosion in a Damascus suburb where government troops have been battling rebels, according to the Syrian Observatory for Human Rights.

The explosion occurred at a military housing unit in the Syrian capital city's suburb of Qudsaya, the rights group said.

Fierce clashes were also reported in Yarmouk Camp, home to the largest Palestinian community in Syria, the LCC said.

Yarmouk Camp has been the scene of on-again, off-again intense fighting in recent months, with government forces shelling rebels and firing on them from helicopters, according to opposition reports.

CNN is unable to independently confirm reports of casualties or violence because the Syrian government has severely restricted access by international journalists.

The Local Coordination Committees of Syria said Thursday's death toll across the country stood at 72, with 31 dead in Damascus and its suburbs and 26 of those killed in Aleppo. The Republican Guard deaths were not part of the group's count.

Russia: Terror tactics in Aleppo "immoral"

Russia denounced a wave of explosions that left dozens dead in the flashpoint city of Aleppo, Syria's largest city, as "immoral" and "inadmissible."

The comments by the Russian Foreign Ministry came as the al Qaeda-linked al-Nusra Front claimed responsibility for Wednesday's bombings that killed at least 40 people and wounded 90 more. The group said the strikes at a popular square were carried out by suicide bombers who drove explosives-laden vehicles and by gunmen disguised as Syrian security forces.