By Khaled Yacoub Oweis and Mariam Karouny
AMMAN/BEIRUT, July 27 (Reuters) - President Bashar al-Assad's artillery pounded rebel-held areas around Aleppo on Friday, preparing the ground for an onslaught on Syria's biggest city where the United States has said it fears a "massacre" may be imminent.
Opposition sources said the shelling, which follows intensive ground and air bombardment of the city itself, was an attempt to stop fighters from resupplying rebel units inside Aleppo.
"They are shelling at random to instil a state of terror," said Anwar Abu Ahed, a rebel commander outside the city.
The battle for Aleppo, a major power centre that is home to 2.5 million people, is being seen as a potentially game changing turning point in the 16-month uprising against Assad that could give one side an edge in a conflict where both the rebels and the government have struggled to gain the upper hand.
A rebel commander said insurgents had attacked a convoy of Syrian army tanks heading towards the city, as the government continued to redeploy forces from other parts of the country to bolster its forces there.
The fate of Syria itself - an ethnically fragmented nation of 22 million people - is likely to determine the future of the wider region for years to come amid fears that its own sectarian tensions could spill across its porous borders.
The U.S. State Department said credible reports of tank columns moving on Aleppo, along with air strikes by helicopters and fixed-wing aircraft, represented a serious escalation of Assad's efforts to crush his opponents.
"This is the concern: that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," Victoria Nuland, a spokeswoman for the State Department, said.
As the remaining residents of Aleppo braced themselves for more bloodshed, General Robert Mood, the outgoing head of the U.N. monitoring mission, told Reuters he thought Assad's days in power were numbered.
"In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall," the Norwegian general, who left Damascus on July 19, said.
Navay Pillay, the United Nations human rights chief, said a pattern had emerged as Assad's forces resorted to shelling, tank fire and door-to-door searches.
"All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city," Pillay said in statement.
Government troops stationed on the outskirts of the city unleashed barrages of heavy-calibre mortar rounds on its western neighbourhoods, while Russian-built MI-25 helicopter gunships struck in the east, opposition activists in the city said.
The heavy fighting follows an audacious bomb attack that killed four of Assad's closest lieutenants in Damascus on July 18, a development that led some analysts to speculate that the government's grip was slipping.
In the first reported casualty on Friday, a man of about 60 wearing a traditional white prayer outfit was killed near a park in Aleppo. His body was placed in a mosque pending identification.
On Thursday, thirty-four people were killed in and around Aleppo, according to opposition activists.
"The rebels have so far been nimble, and civilians have mostly been the victims of the bombardment," said activist Abu Mohammad al-Halabi, speaking by phone from the city.
Majed al-Nour, another activist, said rebels had attacked a security outpost in the neighbourhood of Bustan al-Joz, which is close to Aleppo's city centre, on Thursday.
"The rebels are present in the east and west of the city, and have a foothold in areas of the centre. The regime forces control the entrances of Aleppo and the main thoroughfares and commercial streets and are bombarding the residential districts that fell into rebel hands," he said.
Nour said tens of thousands of people had fled Aleppo to nearby northern rural regions close to Turkey.
HELICOPTERS OVER DAMASCUS
In Damascus on Friday, four helicopters flew over southern areas of the capital, firing heavy machine guns into the districts of Hajar al-Aswad and Tadamon as well as the Yarmouk Palestinian refugee camp, a resident said.
"I can see two above me right now, heading towards Hajar al-Aswad," she said by telephone, the pounding of guns audible in the background. Helicopters were flying low and appeared to be targeting specific buildings.
With U.N. Security Council resolutions for sanctions against Syria vetoed by Russia and China for a third time last week, the United States has said it is stepping up assistance to Syria's fractured opposition, though it remains limited to non-lethal supplies such as communications gear and medical equipment.
Reuters has learned that the White House has crafted a presidential directive, called a "finding," that would authorise greater covert assistance for the rebels, while stopping short of arming them.
It is unclear whether President Barack Obama has signed the document, a highly classified authorisation for covert activity.
A Syrian parliamentarian from the northern province of Aleppo said on Friday she had defected to Turkey, becoming the first member of the rubber-stamp assembly elected in May which is dominated by Assad's Baath Party to defect.
"I have crossed to Turkey and defected from this tyrannical regime," Ikhlas al-Badawi told Sky News Arabia.
One of the most senior figures to defect from Assad's inner circle, Brigadier General Manaf Tlas, has put himself forward as someone who could help unify the opposition inside and outside Syria to help steer a power transfer.
Tlas visited Saudi Arabia before going on to Turkey where he met Foreign Minister Ahmet Davutoglu. Turkey, a former ally of Assad and now one of his fiercest critics, has a heavy strategic stake in shaping any post-Assad leadership.
Meanwhile, a source close to the mediation effort told Reuters on Friday that international mediator Kofi Annan was still trying to forge a political solution to the Syria crisis despite being made a scapegoat for the failure of the two sides to agree. (Additional reporting by Tabassum Zakaria, Matt Spetalnick and Andrew Quinn in Washington; Writing by Giles Elgood; Editing by Andrew Osborn)