Syrian president Bashar al-Assad has warned that his country is in a "real state of war", directing his government take all necessary measures to ensure victory.

"We live in a real state of war from all angles," Mr Assad told a new cabinet he appointed overnight.

"When we are in a war, all policies and all sides and all sectors need to be directed at winning this war."

Mr Assad snubbed countries that have been calling for him to step aside, saying the West "takes and never gives and this has been proven at every stage".

Syria has been in the grips of deadly violence that has claimed more than 15,000 lives, monitors say, since an uprising erupted in March 2011.

On Saturday Mr Assad issued a decree forming a new government, less than two months after controversial parliamentary elections boycotted by the opposition.

The president's latest remarks came as the United States called him "desperate" and said he was slowly losing his grip on power, citing recent defections of army officers and soldiers to neighbouring Turkey and Jordan.

"Clearly, Bashar al-Assad has been slowly - too slowly - losing his grip over his country," White House spokesman Jay Carney said on Air Force One.

"I would note that recent high-level military defections to Jordan and Turkey are another testament to the regime's loss of control over the situation in Syria."

"It is clear, however, that Assad is desperate to hang on to power at all cost, as evidenced by his continued use of air power and Shabbiha [militia] gangs."

A steady stream of high-ranking military officers have defected to Turkey in recent days, and several pilots have made their way to Jordan.

'Imminent threat'

'Not a lamb': Recep Tayyip Erdogan addresses his parliamentary party in Ankara. ( AFP: Adem Altan )

Meanwhile, Turkish prime minister Recep Tayyip Erdogan branded Syria an imminent threat and vowed to retaliate for Syria shooting down a Turkish warplane last week.

Mr Erdogan, who fell out with Mr Assad after he dismissed his advice to allow reforms, said Turkey was no warmonger.

"Our rational response should not be perceived as weakness, our mild manners do not mean we are a tame lamb," he told a meeting of his parliamentary party.

"Everybody should know that Turkey's wrath is just as strong and devastating as its friendship is valuable."

The warning reflected increased tensions not only on the Mediterranean coast, where the F4 Phantom was shot down last Friday, but on a long common land border criss-crossed by rebel groups.

Turkey insists the plane was in international air space at the time, but Syria says it was in Syrian air space.

Mr Erdogan said Turkey had now changed its rules of engagement and would treat any further Syrian threat as a military target.

"Every military element approaching Turkey from the Syrian border and representing a security risk and danger will be assessed as a military threat and will be treated as a military target," he said.

NATO member states, summoned by Turkey to an urgent meeting in Brussels, joined Turkey's condemnation of the incident, although Russia - as an ally of Syria - said it should not be seen as a provocation because it was not intentional.

Rebels' base

Turkey is the base for the rebel Free Syrian Army (FSA) and shelters more than 30,000 refugees - a number Mr Erdogan worries could rise sharply as fighting spreads. Rebel soldiers move regularly across the border and defectors muster inside Turkey.

Fighting has often moved very close to the frontier and could under the new rules of engagement draw Turkish military reaction, especially if Syrian forces pursue rebels.

Syria said on Sunday it had killed several "terrorists" infiltrating from Turkey.

In Syria itself, Damascus suburbs were gripped by the worst fighting in the capital since the uprising against Assad began 16 months ago.

The capital city had long been seen as a bastion of support for the president.

Video posted by activists showed heavy gunfire and explosions. Blood pooled on a pavement in Qudsiya suburb and a blood trail led to a building to where one casualty had been dragged. A naked man writhed, his body pierced by shrapnel.

The Syrian state news agency SANA said insurgents had blocked the old road from Damascus to Beirut.

Dozens of them were killed or wounded and others arrested it said. Government forces also seized rocket launchers, sniper rifles, machineguns and a huge amount of ammunition, it said.

Meanwhile, the United Nations says its mission to Syria will remain suspended for now because conditions on the ground are still too dangerous.

ABC/Reuters