US troops began arriving in Turkey on Friday to man Patriot missile batteries against threats from neighbouring Syria, where the 21-month conflict between the regime and rebels has escalated.

Syrian air and ground forces were pounding insurgents dug in outside Damascus in a ferocious offensive a day after a car bomb in the north of the city killed at least 11 people, according to the Syrian Observatory for Human Rights.

The arrival of the US personnel specialised in the six Patriot systems to be deployed on the Turkey-Syria border under a NATO agreement has highlighted fears that Syria's civil war could suck in other nations in the region.

Cross-border fire has already erupted several times in recent months from combatants in Syria into Turkey, Lebanon and the Israeli-occupied Golan Heights.

The United States last month also expressed concerns that there were signs Syrian President Bashar al-Assad could be preparing to use chemical agents in missiles or aerial bombs as a last-ditch measure against insurgents.

The US military's European Command (EUCOM) said on Friday that the troops being sent to Turkey's Incirlik air base would swell to 400 within days to support the two US Patriot batteries being supplied by America.

Germany and the Netherlands will supply the four other Patriot batteries under the NATO agreement struck at Turkey's request and described as purely defensive.

"The forces will augment Turkeys air defence capabilities and contribute to the de-escalation of the crisis along the Alliance's border," the EUCOM said.

Syria's chief ally Iran, however, has called the Patriot deployment "provocative," seeing it as a blunting of its own offensive capabilities.

Ankara has responded by telling Tehran to use its clout with Assad to resolve the civil war in his country.

That conflict has worsened in the past six months as Assad has ordered warplanes and heavy artillery to blast rebels who hold great swathes of Syria's countryside, especially in the north.

The United Nations this week said 60,000 people have died since the rebellion began in March 2011. Its figures showed average daily fatalities have multiplied since mid-2012, correlating with the increased use of regime air power.

On Friday, fighter-bombers hit Duma, northeast of Damascus, and artillery was shelling the southwestern Daraya neighbourhood which the rebels have held for weeks, the Observatory said.

Troop reinforcements were being sent to Daraya, the British-based group added.

The offensive was being waged a day after a car bombing in the Damascus district of Massaken Barzeh, mostly inhabited by members of Assad's Alawite minority, killed at least 11 people, the watchdog said.

They were among at least 191 people killed on Thursday, including 99 civilians, it said, adding that fighting in Damascus and its outskirts accounted for 87 deaths.

Nationwide on Friday at least 115 people died, among them 66 civilians, according to the Observatory.

It also said rebels killed a relative of political security chief Rustom Ghazali, a key regime figure, wounded another and kidnapped a third in the southern province of Daraa.

State television also reported the attack, without identifying the victims.

A pro-regime journalist working for Dunya TV was also shot dead in Aleppo while reporting there, the channel announced.

People demonstrated across Syria in solidarity with the central city of Homs, where conditions are dire in areas besieged by regime forces.

Meanwhile, Damascus slammed as "biased" a UN report released on December 20 that called the conflict "overtly sectarian in nature."

The foreign ministry accused the UN of a "lack of professionalism" in producing its report, and said any sectarian dimensions to the conflict were because of foreign support for "armed groups," state news agency SANA said.