At least 54 people were killed when a Syrian air strike hit a fuel station in the northern province of al-Raqqa on Thursday, a British-based violence watchdog said, an area of heavy fighting between government and rebel forces.
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The Syrian Observatory for Human Rights, which has a network of activists across Syria reporting on government violence during the 18-month-old revolt, cited an activist in the region saying that more than 110 people were dead or wounded.
A video published by activists, said to be from al-Raqqa, showed black clouds of smoke rising from the wreckage of the petrol station as bewildered residents examined the scene following the attack by a Syrian air force jet.
It came a day after insurgents seized a border crossing with Turkey some 30 km (18 miles) away on the northern fringes of al-Raqqa province, consolidating their grip on a frontier through which they ferry arms for battle.
It was impossible to verify the authenticity of the video, and most foreign journalists are barred entry into Syria, making varying accounts of events difficult to confirm.
President Bashar Assad has used helicopters and fighter jets to fire at and bomb parts of the country where insurgents have been operating, including residential districts of the capital and Syria's main cities.
Assad's forces have targeted petrol stations in rural towns and villages and along main roads to deprive rebels of fuel. Civilians have set up smaller, discreet fuel outlets.
Activists say more than 27,000 people have been killed in a conflict that began with peaceful street protests and mushroomed into civil war after Assad tried military force to stamp out the unrest. Last month was the bloodiest yet.
Earlier on Thursday, Syria's information ministry said that a Syrian military helicopter that crashed near the capital had clipped the tail of a Syrian Arab Airlines passenger plane, but the 200 people on board escaped unharmed.
"The helicopter struck the tail of the plane ... The control tower at Damascus airport confirmed that the plane landed safely at Damascus airport and all 200 passengers are in good health," a statement published on the state news channel Syria TV said.
Rebellious Damascus District Raided
On the ground, security forces surrounded and raided a rebellious southern district of Damascus, arresting more than 100 people, and activists said several others were shot dead.
An opposition activist called Abu Salam, who lives in the Yarmouk district where rebels have been hiding out in recent days, told Reuters that many residents were trapped.
He said tanks and soldiers had sealed all the entrances and hundreds of soldiers were searching the area on foot and on trucks mounted with heavy machineguns.
"We are hiding in our homes. I am afraid to leave the house so I am sitting here waiting to see if they reach my street, if I will be arrested or shot dead," he said, adding that at least three people, two men and a young women, were shot dead when soldiers saw them running out of a park on Thursday morning.
He said another five rebels found hiding were executed.
A resident who toured Yarmouk a day earlier said rebel fighters, who have been flushed out of many surrounding districts, had moved into a southern section of the district and come under intense army bombardment overnight.
Assad has long maintained that foreign-backed militants have been leading the revolt, which started with peaceful protests but turned violent as the army fired on demonstrations and rounded up thousands.
State media said on Thursday that soldiers had killed 100 Afghan "terrorists" in the northern city of Aleppo. Rebels in the area said that report was government propaganda, saying that the district of Bustan al-Qasr -- where the attack supposedly took place -- has not been entered by Assad's troops.
Divided world cannot halt violence
Global and regional powers have failed to halt the violence. Iran and Russia have backed Assad while the United States and European allies want him toppled but shrunk from intervening in a conflict steeped in ethnic and sectarian rivalries that could spill over Syria's borders and inflame the wider Middle East. 
Iraq denied on Thursday a Western intelligence report that said Iranian aircraft and trucks had transported weapons and military personnel through Iraq to Syria to help Assad corral the uprising and Belarus denied trying to sell weapons to Syria.
"Iraq has confirmed that it will never be involved or helping or allowing any shipment via its air space or land to Syria," Iraqi government spokesman Ali al-Dabbagh told Reuters.
The allegation, reported by Reuters on Wednesday, said arms transfers were organized by the Islamic Revolutionary Guards.
Syria's upheaval is a political headache for Iraq's Shi'ite Muslim-led government. Close to Assad's ally, Shi'ite Iran, Baghdad has resisted joining Western and Gulf Arab calls for the authoritarian leader, whose family has ruled for 42 years, to bow out while also calling for a reform process in Syria.
Baghdad's core concern is that a precipitous fall of Assad would fracture Syria along sectarian lines and yield a hostile, hardline Sunni Muslim regime that could stir up Iraq's combustible Sunni-Shi'ite communal mix.
Belarus denied trying to sell weapons to Syria and violating a UN Security Council resolution after the United States imposed sanctions on a Belarusian state-owned firm.
"All the accusations of the American side... have no basis and are untrue," Foreign Ministry spokesman Andrei Savinykh said after the United States imposed sanctions on a Belarusian state-owned firm.
The Syrian rebels are being armed by Sunni Muslim states like Saudi Arabia and receive other supplies and diplomatic support from the Western powers and Turkey.