Tensions mounted on both sides of the Syria-Turkey border on Tuesday, with open talk of war in Damascus while its neighbour to the north said military action would follow any future violations of its territory.

"We live in a real state of war from all angles," Syrian President Bashar al-Assad told his cabinet on Tuesday, according to Reuters. "When we are in a war, all policies and all sides and all sectors need to be directed at winning this war."

Assad's remarks came amid heavy fighting outside the capital between rebel and government forces, and strong words from Turkey in response to the downing of one of its jets on Friday by Syrian forces.

"Any military element that approaches the Turkish border from Syria … will be regarded as a threat and treated as a military target," said Prime Minister Recep Tayyip Erdogan.

Syria insists that the Turkish plane violated its airspace. Turkey maintains that although the unarmed RF-4E reconnaissance jet had unintentionally strayed into Syria's airspace, it was in international airspace when it was brought down.

Tuesday also saw Russia enter the war of words, albeit with a call for calm and restraint. Russia's Foreign Ministry said the loss of Turkey's jet should not be seen as a "provocation" by Syria, its close ally in the region.

"It is important that what happened is not viewed as a provocation or a premeditated action," said the ministry in a statement, according to Reuters.

"We believe that the best course of action is restraint and constructive interaction between the Turkish and Syrian sides in order to clarify all the circumstances of the incident."

Russia also reportedly warned world leaders against using the incident to push for stronger action against Syria.

The head of the NATO military alliance called the downing of the jet unacceptable on Tuesday, shortly after Turkey briefed NATO's North Atlantic Council in discussions held under Article 4 of NATO's founding treaty, which allows a NATO member to request consultations if its security has been threatened.

NATO Secretary General Anders Fogh Rasmussen said the alliance condemns it "in the strongest terms," and expressed solidarity with Turkey, but made no mention of retaliatory action.

Erdogan said Syria shot down the unarmed plane in international airspace in a "deliberate" and "hostile" act and without warning. He said border violations in the region were not uncommon, and that Syrian helicopters had violated Turkish airspace five times recently, without Turkish response.

"No one should be deceived by our cold-blooded stance," Erdogan said. "Our acting with common sense should not be perceived as a weakness."

The downing of the jet has aggravated tense ties between the two neighbours. Turkey has repeatedly called on Assad to step down as 33,000 Syrians have sought refuge in Turkey, fleeing a government crackdown on a popular uprising.

Erdogan said as much as Turkey's friendship is valuable, Turkey's "wrath is as much violent and crushing."

"We will continue to be a burning torment for circles who have adopted a hostile attitude toward Turkey," he said.

The public anger in Turkey is largely muted, and Huseyin Celik, a senior member of Erdogan's ruling party, said the party grassroots is against going to war.