Turkish Prime Minister Recep Tayyip Erdogan warned Friday that Turkey and Syria are “not far” from war as the two countries exchanged mortar fire across their border for a third consecutive day.

The belligerent remarks and the continued shelling sustained fears that the countries are drifting toward war more than a year after Turkey threw its support behind Syrian opposition demands for the ouster of President Bashar al-Assad, reversing a decade of warming relations.

Turkish officials said that Turkey’s military fired mortar rounds into Syria on Friday evening after a Syrian shell crashed into farmland in the southern province of Hatay, more than 200 miles from the site of shelling by Syria on Wednesday that killed five people in the Turkish town of Akcakale.

That attack triggered almost immediate Turkish retaliation against Syria in the most serious cross-border incident since the Syrian uprising evolved into an armed rebellion last year. Although it was not the first time shells fired in Syria had strayed into Turkey, Turkish officials said this strike was different because it involved more than six shells fired simultaneously at the same area, and because civilians died. The shell fired into Turkey on Friday caused no casualties, and Syria did not comment on the incident.

In a speech in Istanbul on Friday, Erdogan made it clear that Turkey is prepared to go to war to defend its territory, a day after the Turkish parliament authorized the government to carry out military operations beyond its borders.

“We are not interested in war, but we’re not far from it either,” Erdogan said, adding, “When they say, ‘If you want peace, prepare for war,’ it means that when the time comes, war becomes the key to peace.”

The flare-up of tensions along the Turkish border has intensified fears that the Syrian revolt, which has already polarized the region along sectarian lines, could spill beyond its territory. Turkey and the Persian Gulf’s Sunni leaders have lined up behind the rebels, while the Shiite-led governments of Iran, Lebanon and Iraq support Assad.

In another indication of the ways Syria’s neighbors in the region are being drawn into the conflict, the Free Syrian Army battalion that claimed in August to have kidnapped 48 members of the Iranian Revolutionary Guard Corps warned that it would begin executing the hostages unless the Assad government halts attacks on civilian areas within 48 hours.

In a video aired on the al-
Arabiya network
, a uniformed rebel claiming to represent the Baraa battalion threatened to kill one Iranian hostage for every Syrian killed after the deadline passes. There was no immediate comment from Iran, which has acknowledged that at least some of the men were retired members of the elite Iranian unit and has also admitted that it has sent military advisers to help the government in Syria.

The East Ghouta suburb of Damascus, where the battalion is based, has been the target of an intensified government offensive in recent days, but the rebels have fought back. On Friday, they claimed to have shot down a government helicopter and posted a video on YouTube showing a burning helicopter spiraling through the sky to the ground. On Thursday, the rebels also claimed to have captured a significant air defense base in the area complete with sophisticated air-defense systems and surface-to-air missiles.

Government forces also unleashed an unusually heavy attack on the central city of Homs, where rebel-held neighborhoods have been besieged and regularly shelled by loyalist forces for months. The Britain-based Syrian Observatory for Human Rights said it was the heaviest bombardment there in months. The Local Coordination Committees, an opposition group, said seven people were killed, among 40 who died nationwide in government attacks.

Ahmed Ramadan contributed to this report.

Interactive: Recent events in Syria