Warplanes pounded rebel-held areas of Homs in the Syrian city's heaviest onslaught for months on Friday, monitors said, as Turkey reportedly returned cross-border shellfire for the second time this week.

The clashes came as the UN condemned the Syrian army's deadly shelling of a Turkish border town on Wednesday, and "terrorist" car bombs that killed almost 50 people in the war-torn country's second city Aleppo on the same day.

The Syrian Observatory for Human Rights said the bombardment of Khaldiyeh district was the most intense of Homs in five months, and the first time the regime has deployed fighter jets against the city.

Rebels hit back by downing a regime helicopter gunship in Damascus province and seizing an army checkpoint outside the eastern city of Deir Ezzor, killing at least 12 soldiers, said the Observatory.

"Allahu akbar! (God is greatest). The helicopter is falling!" a youth yelled as an aircraft was seen tumbling from the sky in flames on an amateur video that activists posted on the Internet.

The Observatory reported eight civilians and one rebel killed in Homs city and province, updating its Friday death toll to 95 nationwide -- 37 soldiers, 35 civilians and 23 rebel fighters.

It said six civilians and 11 rebels were killed in Aleppo.

The insurgents have occupied health facilities and schools as makeshift bases, the journalist said, adding they move around the area through holes in the walls of buildings.

There also were signs of strain from troops in the city, with one officer admitting "the battle for Arkoub is as tough as the struggle for Hanano" last month, when rebels took control of a fortress-like compound for three days.

Conscripts fighting for the regular army spoke in different accents from across the country.

Flashing a V-for-victory sign, one young soldier said he missed his home in Homs province. "My people in Homs haven't seen me in three months," he lamented.

Despite the relentless violence, thousands of people demonstrated across Syria calling for the arming of the rebel Free Syrian Army and condemning the world's inaction, the Observatory reported.

The uprising, which began in March 2011 with pro-reform protests that were brutally crushed, has since turned into a civil war pitting mainly Sunni rebels against President Bashar al-Assad's minority Alawite-dominated regime.

Activists say more than 31,000 people have died in the conflict.

The world community has expressed fear the war could spill over into other countries in the region, exacerbated by exchanges of shelling between Syria and Turkey on Wednesday and Thursday.

Turkey had demanded strong UN action after Syrian fire killed five people in the border town of Akcakale on Wednesday, including a mother and her three children.

And on Friday, a Syrian shell hit the Turkish town of Altinozu in Hatay province, triggering an immediate response fire from Turkish forces at around 1630 GMT, said NTV news channel.

There were no immediate reports of any casualties.

'We want weapons, not words!'

The UN Security Council issued a toughly worded statement in response to Syria's initial shelling, although it was a rung down from a formal resolution.

"The members of the Security Council condemn in the strongest terms the shelling by the Syrian armed forces," it said.

Saying the shelling "highlighted the grave impact the crisis in Syria has on the security of its neighbours and on regional peace," the statement "demanded that such violations of international law stop immediately and are not repeated."

A statement agreed on Friday by the 15-nation council condemned the Aleppo bomb attacks "in the strongest terms" and expressed condolences to families of the victims "of these heinous acts."

On Friday, Turkish premier Recep Tayyip Erdogan warned Syria it would pay a big price for any more attacks and reiterated Turkey does not want war but is ready to act if threatened.

"I am calling once more on the Assad regime and its supporters: Don't dare to test Turkey's patience," he told a crowd in Istanbul.

Prior to the latest shellfire, the border region between Turkey and Syria had been quiet on Friday, although the Turkish military had amassed tanks and anti-aircraft missiles.

Inside Syria, protesters took to the streets of several cities and towns after being urged to turn out under the rallying cry: "We want weapons, not words, to protect our children from the killing".

"The people demand the arming of the FSA (Free Syrian Army)!" hundreds of men chanted outside a white mosque in Halfaya, Hama province, while holding the flag of the revolt.

Meanwhile, a rebel commander warned his fighters would execute dozens of Iranians they kidnapped inside Syria in August unless the army withdraws from the embattled Eastern Ghuta area of Damascus province.

"We gave the regime 48 hours starting yesterday to withdraw completely from the Eastern Ghuta area," Abul Wafa said via the Internet. "We also have other secret, military demands. If the regime does not fulfill them we will start finishing off the hostages."