Syrian rebels on Thursday shelled an air base being used by regime forces to pound the northern city of Aleppo, as a rights watchdog reported 43 people killed in a raid near Damascus.
"Menagh military airport was bombarded on Thursday morning by a tank captured previously by the rebels," the Syrian Observatory for Human Rights said of the base 30 kilometres (18 miles) northwest of the country's commercial capital.
An AFP reporter who witnessed the bombardment said rebels told him it was "an attack to take this airport being used by helicopters and planes that are firing on Aleppo."
The United Nations confirmed on Wednesday that rebels battling President Bashar al-Assad's regime now had heavy armour, and that its military observers had seen the Syrian military use a fighter jet to attack rebels in Aleppo.
AFP correspondents on the ground have reported that rebels have captured a number of tanks, and some armoured units have defected with their vehicles.
It is difficult to get an overall picture of the situation inside Aleppo itself because of a lack of independent sources and restrictions on journalists.
The Observatory and an activist said mobile phone and Internet services in Aleppo have been cut since Wednesday, and a security source in Damascus told AFP such cuts are "generally the precursor to a major military offensive."
Thursday's air base assault comes after US President Barack Obama was reported to have signed a covert document authorising US support for the rebels.
The directive was contained in a "finding" -- a device authorising clandestine action by the Central Intelligence Agency, NBC and CNN said, citing unidentified sources.
White House officials declined to comment, but did not specifically rule out the idea that Washington was providing more intelligence support to anti-Assad forces than had previously been made public.
US Defence Secretary Leon Panetta was in Jordan on Thursday for talks on Syria.
"Both of our nations share concerns about what is happening in Syria and the impact that that could have on regional stability," he told reporters.
The Observatory said a security forces raid southwest of Damascus killed 43 people, some of whom were tortured and executed.
"Regime forces entered the Jdaidet Artuz district on Wednesday and arrested around 100 young people who were taken to a school and tortured," it said.
"On Thursday morning after the operation the bodies of 43 people were recovered. Some of them had been summarily executed."
The Observatory had reported on Wednesday 28 civilians killed in the raid.
A resident of neighbouring Artuz said the army had shelled the village from Jdaidet Artuz.
"There's nobody. Not one shop is open; the houses have been deserted by their inhabitants fearing violence -- everyone has fled," the resident said.
Pro-regime newspaper Al-Watan on Thursday said security forces were "hunting down terrorist groups" in Damascus province.
Nationwide, 163 people were killed on Wednesday -- 98 civilians, 20 rebels and 45 soldiers -- said the Observatory, which has estimated that more than 20,000 people have died since mid-March last year.
Assad said on Wednesday the army was fighting for Syria's future.
"The army is engaged in a crucial and heroic battle... on which the destiny of the nation and its people rests," the official SANA news agency quoted him as saying in a speech marking armed forces day.
Washington mocked Assad for not delivering his speech in public.
"We think it's cowardly quite frankly to have a man who's hiding out of sight be exhorting his armed forces to continue to slaughter the civilians of his own country," said a US State Department spokesman, Patrick Ventrell.
In London, British Prime Minister David Cameron and Russian President Vladimir Putin were to discuss Syria on Thursday.
Britain has strongly criticised Moscow's refusal to back UN Security Council action against the Damascus regime, and the Kremlin said Putin would staunchly defend Russia's position on the crisis.
UN spokesman Martin Nesirky stressed on Wednesday that UN leader Ban Ki-moon wants united international pressure on both sides.
He said pressure should be brought to bear on "not just the Syrian government forces -- who of course bear the lion's share of the responsibility for what is happening -- but also on the opposition forces, to ensure that they do heed the calls, that they do stop the fighting."
On Friday, the UN General Assembly will vote on a largely symbolic Arab-drafted resolution calling on Assad to stand down.
The Free Syrian Army's military chief in Aleppo, Colonel Abdel Jabbar al-Oqaidi, said the rebels had "thousands" of fighters in Aleppo.
"The regime says it is fighting 'terrorist groups.' We tell the regime that we will chase them because they are the terrorists," Oqaidi told AFP.
"We will go after them in the whole of Aleppo, until the city is liberated."
The United Nations says that some 200,000 of the city's estimated 2.7 million population have fled their homes, many of them taking refuge in schools and other public buildings.
Three million Syrians need food, crops and livestock assistance, the UN's Food and Agriculture Organisation said, citing a survey by the United Nations and the Syrian government.
The FAO said figure included 1.5 million Syrians who "need urgent and immediate food assistance over the next three to six months, especially in areas that have seen the greatest conflict and population displacement."