ISTANBUL, Turkey — Turkey renewed cross-border attacks in Syria on Thursday, even as a government official said the country has “no interest in war with Syria.”

The cross-border attacks began Wednesday when Turkey fired on Syrian government targets in retaliation for shelling that struck one of its border towns and killed at least five people.

It was the first time that Turkey fired into Syria during the 18-month-long crisis there.

The two countries once enjoyed a cozy bilateral relationship that saw visa-free travel and booming trade between them. But relations ruptured as Syrian President Bashar al-Assad’s government continued its bloody and unrelenting crackdown on anti-government demonstrators.

Now, the cross-border attacks are raising fears the spillover from Syria’s civil war could ignite a wider regional conflict. The Syrian government has said it is investigating Wednesday’s incident.

U.N. Secretary-General Ban Ki-moon warned the strikes demonstrate “how Syria’s conflict is threatening not only the security of the Syrian people but increasingly causing harm to its neighbors.”

The shelling into Syria struck a chord in Turkey. 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.”

On Thursday, Turkish parliament was set to consider a resolution to pre-emptively strike its neighbor should it pose a threat.

“This circumstance has reached a point that constitutes serious threat and risk to our national security. Therefore, it has become necessary to be able to respond to further risks and threats in a timely and immediate manner,” according to a resolution sent to parliament by Turkish Prime Minister Recep Tayyip Erdogan.

The Turkish military began shelling Syrian military sites in Tal Abyad, about 10 kilometers (6.2 miles) from the Turkish border, on Thursday, the opposition Local Coordination Committees of Syria reported.

The artillery bombardment “continued for a while last night — very early this morning,” a senior Turkish official, who spoke on condition of anonymity, told CNN. The official, who was not authorized to speak to the media, said the shelling was near the border town that was targeted Wednesday.

The official could not provide details about Syrian casualties.

But the London-based Syrian Observatory for Human Rights said a number of Syrian security forces were killed.

“Turkey has retaliated to yesterday’s incident without declaring war on Syria. Political, diplomatic initiatives will continue,” Ibrahim Kalin, the foreign policy adviser to Erdogan said Thursday in a posting on Twitter.

He also said: “Turkey has no interest in a war with Syria. But Turkey is capable of protecting its borders and will retaliate when necessary.”

The resolution before parliament calls for authorizing the government to send Turkish forces “to foreign countries, with the administration determining the where, the scope, the numbers and the time of such deployment.”

The resolution points to a series of aggressive acts against Turkey by Syria beginning September 20, the day fighting between rebels and Syria government forces near the border left three Turkish civilians wounded and temporarily forced the closure of area schools and farms.

“This should not be seen as a war resolution,” said Omer Celik, of Turkey’s ruling Justice and Development Party. “This is a resolution protecting the national security and sovereignty of Turkey.”

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.

Syria, meanwhile, has accused Turkey of arming and funding Syrian rebels.

In a statement, Ban warned late Wednesday the “ongoing militarization of the conflict in Syria is leading to tragic results for the Syrian people.”

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 London-based 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.

Russia: Terror tactics in Aleppo “immoral”

Russia, a Cold War-era ally of Syria, 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 explosive-laden vehicles and armed gunman disguised as Syrian security forces.

The see-saw fight for Aleppo, once considered an al-Assad stronghold, has continued nearly unabated since July, though the number of casualties has steadily increased.

“Those behind the attacks and those who carried them out must be found and punished,” the Foreign Ministry said in a post on its official Twitter account.

It also said: “Supporting those who commit crimes like these is immoral and inadmissible.”

Russia, along with China, has repeatedly blocked U.N. Security Council efforts to try to end the carnage. Russia’s foreign minister has said the conflict must be decided by the Syrian people.

While most of the Western members of the Security Council condemned the border attack, Russian Foreign Minister Sergey Lavrov had sharp words for their failure to condemn the attack in Aleppo.

He said he regrets the “Security Council once again failed to condemn terror acts in Syria.”

Lavrov also said Damascus should promise publicly to prevent armed incidents on the border with Turkey.