Bolstered by a declaration of support from NATO, Turkey warned Syria on Tuesday that it would retaliate if Syrian forces approach its southern border, signaling a significant escalation of tensions between the neighbors after the downing of a Turkish jet.

The warning coincided with a strong condemnation of Syria by NATO, which waded into the Syria crisis for the first time with a statement calling the attack on the plane “unacceptable” and stressing that the alliance stands with Turkey “in the spirit of strong solidarity.”

The Turkish threat, along with NATO’s unequivocal declaration of support, raised the risk of a confrontation along the 550-mile Turkish-Syrian border, which is already a focus of Syrian efforts to crush the 15-month-old revolt against President Bashar al-Assad’s rule. Large swaths of the border region have fallen under rebel control, and the Syrian government routinely launches attacks in the area.

Turkish Prime Minister Recep Tayyip Erdogan said Tuesday in a strongly worded speech to members of parliament in Ankara that “any military element that approaches the Turkish border from Syria, posing a security risk or danger, will be regarded as a threat and treated as a military target.”

“This incident shows that Syria has become an open threat to Turkey, and so we have come to a brand-new stage,” he said.

Hours after he spoke, Turkish media reported that tanks and other heavy military equipment had been dispatched to the Syrian border area from a base in southeastern Turkey.

Erdogan made it clear that Turkey plans no immediate military retaliation for the downing of the jet on Friday. But by changing its rules of engagement along the border, Ankara is putting Syria on notice that it can no longer operate there with impunity. The action could curb Syria’s capacity to hunt down rebels in the northern province of Idlib, regarded as one of their strongholds.

Smuggling routes between Turkey and Syria are used by the rebel Free Syrian Army, which is based in a refugee camp in southern Turkey, to secure supplies of weaponry and money. The cross-border flow of rebels and refugees is one of the many sources of friction between Ankara and Damascus.

There have been several instances in recent months in which Syrian troops have fired into Turkey to target fleeing refugees or rebels, and Syrian helicopters have strayed into Turkish territory at least five times, Erdogan said.

Assad countered Erdogan’s comments with a defiant speech in which he declared that Syria is “in a state of real war.”

“When we are in a war, all our policies . . . need to be directed at winning this war,” he told his cabinet, betraying no indication that Syria had been chastened by the tough international response to the downing of the Turkish jet.

Assad has always portrayed the rebellion against his rule as a conspiracy led by the United States and its allies, and his comments suggested that the growing pressure has only affirmed his view.

(Gene Thorp/The Washington Post)

A boost for rebels

Rebels welcomed Erdogan’s remarks, saying that a more aggressive Turkish posture will provide a significant boost.

“The Turkish reaction is serious and firm,” Col. Malik Kurdi, a spokesman for the Free Syrian Army, said by telephone from southern Turkey. “This new situation will strengthen the Free Syrian Army not only in Idlib and the border areas, but in the whole of Syria.”

If the rebels can seize the initiative and push Syrian forces back from the border, the Turkish threat could create some form of buffer zone along the lines that the Syrian opposition has been calling for, said Salman Shaikh, director of the Brookings Doha Center in Qatar.

“That border is not quite a tripwire yet, but certainly it’s a lot more tense than it was a week ago,” he said. “The Turks clearly think Syria is a threat to regional peace and security.”

The Turks may also accelerate a quiet effort gathering pace along the border to provide funding and weapons to the Free Syrian Army. Saudi Arabia, Qatar and the United States are participating in that effort, Shaikh said.

Turkey denies arming the rebels, but Erdogan pledged that he would “offer all possible support to liberate the Syrians from dictatorship.”

Turkey and Syria have offered sharply contradictory versions of what happened when the F-4 fighter jet was brought down over the eastern Mediterranean on Friday. Turkey says the plane was in international airspace when it was struck by a missile; Syria says it was within a mile of the Syrian coast, in Syrian airspace, when it was targeted by antiaircraft machine-gun fire. The jet’s two pilots have not been found.

Although the Turkish jet did briefly stray into Syrian airspace, “this kind of short-term border violation can never be regarded as a pretext for a hostile attack,” Erdogan said.

Erdogan’s sense of betrayal

Erdogan’s speech was sprinkled with references to his deep sense of betrayal as Assad, with whom he had forged a close personal relationship, reneged on promises made to Turkey to implement reforms and ease the government’s harsh crackdown on protesters.

Syria had been a focus in recent years of Turkey’s efforts to project its influence in the Arab world, to the extent that the two countries held joint cabinet meetings.

The rapprochement followed more than a decade of tensions during the rule of Assad’s father; the countries came to the brink of war in 1998 over Syria’s support for the radical Kurdistan Workers’ Party, which Turkey considers a terrorist group.

Erdogan had clearly hoped that his friendship with Assad would help end the crisis. But, he said, “now we see that all our hopes were in vain because he was not speaking the truth. We realized the son was not keeping his promises and was following in the footsteps of the father.”

As Erdogan spoke, Syrian rebels said they had attacked a base belonging to the elite Republican Guard in the Damascus suburb of Hameh, in a sign of the rebels’ growing boldness. Residents of Damascus reported hearing explosions and gunfire, and the Local Coordination Committees, an opposition group, said 33 people were killed in the subsequent bombardment of the area.

Early Wedneday, Syria’s state-run news agency reported that gunmen attacked the headquarters of pro-Assad Ikhbhariya TV station 14 miles south of Damascus, killing three employees.

‘Unacceptable act’

NATO did not propose any measures against Syria during an emergency meeting in Brussels summoned by Turkey. But NATO Secretary General Anders Fogh Rasmussen told reporters that the organization would closely monitor events along the alliance’s southeastern border and would gather to “discuss what else will be done” should another such incident occur.

“It is my clear expectation that the situation won’t continue to escalate,” he said. “What we have seen is a completely unacceptable act, and I would expect Syria to take all necessary steps to avoid such events in the future.”