Turkey has warned Syria to keep its forces away from the countries' troubled border or risk an armed response - a furious reply to the downing of a Turkish military plane last week by the Damascus regime.
Nato backed up Turkey and condemned Syria for shooting down the plane but stopped short of threatening military action, reflecting its reluctance to get involved in a conflict that could ignite a broader war.
Near the capital of Damascus, meanwhile, Syria's elite Republican Guard forces battled rebels in some of the most intense fighting involving the special forces since the uprising against President Bashar Assad's regime began in March 2011, according to activists.
Assad appeared to acknowledge the seriousness of the situation while addressing his new Cabinet on Tuesday in a statement broadcast on Syrian state TV, when he said his country is in a "genuine state of war". Up to now Assad has described the uprising against him as run by terrorists carrying out a foreign agenda.
More than 14,000 people have been killed in the last 15 months. Despite global outrage over the crackdown by the Assad regime, the international response has been focused entirely on diplomacy and sanctions, not intervention, as the violence escalates.
In a speech to parliament, Turkish prime minister Recep Tayyip Erdogan said Syria shot down the unarmed reconnaissance plane in international airspace without warning in a "deliberate" and "hostile" act. "Any military element that approaches the Turkish border from Syria and poses a security risk and danger will be regarded as a threat and treated as a military target," Mr Erdogan said.
He said border violations in the region were not uncommon and Syrian helicopters had violated Turkish airspace five times recently without a Turkish response. The two countries share a 566-mile (910km) frontier.
Turkey's limited response to Friday's incident suggested there was no appetite for a violent retaliation. Still, Mr Erdogan cautioned Syria against testing his resolve."No one should be deceived by our cool-headed stance," he added. "Our acting with common sense should not be perceived as a weakness."
Syrian officials insist the plane violated its airspace, saying a Syrian officer shot it down with anti-aircraft fire after spotting an unidentified jet flying at high speed and low altitude.
Turkey disputes that. Turkey says although the RF-4E jet had unintentionally strayed into Syrian airspace, it was inside international airspace when it was brought down over the Mediterranean. Its two pilots are still missing.