NATO members were holding emergency talks on Syria's downing of a Turkish warplane, as a monitoring group reported fierce clashes on Tuesday around elite Republican Guard posts in suburbs of Damascus.
The United States, meanwhile, denounced the UN's "colossal failure" to protect civilians inside Syria.
The NATO meeting in Brussels was being held after a fuming Ankara told the UN Security Council that Syria's downing of one of its fighter jets last Friday posed a "serious threat to peace and security".
Damascus insists the plane had violated its airspace and "sovereignty."
"The aircraft did not display any hostile attitude or manoeuvre and was flying with its identification systems open. The shooting came without any prior warning," Turkey said in a letter to the Security Council and UN chief Ban Ki-moon and which was obtained by AFP.
Turkey's UN ambassador Ertugrul Apakan said the incident was "a serious threat to peace and security in the region, in the context of the Syrian crisis" but did not call for the world body to act against Damascus.
Syria has defended its downing of the Turkish F-4 Phantom jet.
"The Turkish warplane violated Syrian airspace, and in turn Syrian air defences fired back and the plane crashed inside Syrian territorial waters," foreign ministry spokesman Jihad al-Makdissi said Monday.
"What happened is a gross violation of Syrian sovereignty," he added.
The incident has reignited international concern over the Syria conflict.
European Union foreign ministers meeting in Luxembourg on Monday condemned the attack and announced new measures targeting Syrian government ministries and companies, including a bank and a television channel.
But the ministers also warned of the dangers of a military escalation in the crisis, and praised Turkey's "measured and responsible initial reaction".
NATO members Britain, France and the United States have all condemned Syria, with Britain saying Damascus should not be allowed to act with impunity.
Iran, an ally of Turkey and Syria, called on Tuesday for regional countries rather than outside powers to help resolve the row.
Iran's foreign ministry spokesman Ramin Mehmanparast said that "key players in the region" would be able to prevent the incident escalating into a problem involving other countries.
"We will use our good relationship with the two countries to resolve the issue," he said.
Turkey called the emergency NATO meeting by invoking Article Four of the alliance's founding treaty, which covers threats to members' security.
"The facts in our possession show that our plane was hit by a heat-seeking guided laser missile," Turkey's Vice Prime Minister Bulent Arinc said.
The jet had been "intentionally shot down... in international airspace," he added.
"To target an aircraft in this fashion without any warning is a hostile act of the highest order," he said, adding that Ankara could soon announce a cut in power supplies to Syria.
Later Monday, Arinc accused Syria of having opened fire on a rescue plane searching for the jet's two pilots who are still missing. "Everyone must know that this sort of behaviour will not go unpunished," he said.
On Tuesday, Syrian armed rebel forces and regime army units were locked in fierce clashes around elite Republican Guard posts in the suburbs of Damascus, the Syrian Observatory for Human Rights said.
"Violent clashes are taking place around positions of the Republican Guard in Qadsaya and Al-Hama," eight kilometres (five miles) from central Damascus, the Observatory head Rami Abdel Rahman told AFP.
"This is the first time that the regime uses artillery in fighting so close to the capital," he said. "This development is important because it's the heaviest fighting in the area and close to the heart of the capital."
"These suburbs are home to barracks of troops which are very important for the regime like the Republican Guard. This is also where families of (army) officers live," he said.
The sustained bloodshed in Syria saw Washington lashing out at the United Nations on Monday.
"The situation in Syria represents a colossal failure by the Security Council to protect civilians," Washington's UN ambassador Susan Rice said as the 15-nation body debated the crisis.
"It is a shame that this Council continues to stand by rather than to stand up," she said giving a withering assessment of the situation in Syria.
Rice called for tougher sanctions against Syrian President President Bashar al-Assad, a position backed by Britain.
Inside Syria, the killing saw 95 people including 61 civilians killed on Monday, as the army pounded rebel strongholds and other towns and cities.
According to the Britain-based Observatory, the conflict has cost more than 15,000 lives since it erupted in March 2011.