BEIRUT - Syrian forces stormed the last rebel stronghold in the capital Damascus in tanks and armoured vehicles on Friday, and blasted with artillery and helicopters at Aleppo, where the United Nations said they were preparing a massive assault.
The violence came within hours of former United Nations Secretary-General Kofi Annan quitting as international peace envoy for Syria, underlining the impotence of mediation efforts in the 17-month-old uprising against President Bashar al-Assad.
A senior United Nations official said a long-expected army onslaught to take Aleppo, Syria's largest city and economic hub, was now imminent following a build-up of army reinforcements. The fighting in Aleppo has focused on the Salaheddine district, seen as a gateway to the city for the Syrian army.
Rebels poured into Aleppo in July after being largely driven from the capital Damascus, where they had launched an offensive that coincided with a bomb blast that killed four top security officials. The fighting in the two main cities has intensified the conflict over the past three weeks.
Rebels told Reuters journalists inside Aleppo on Friday that they had captured a large police station after days of clashes. Rebel commander Abu Zaher said fighters had taken several police officers prisoner and seized weapons and ammunition.
Other rebels said heavy fighting was taking place in Saleheddine, the main battleground district, where they estimated 20 civilians had been killed. They say 50 of their fighters have been killed there in the last several days.
U.N. Secretary-General Ban Ki-moon said "acts of brutality" reported in Aleppo could be crimes against humanity. Both sides have accused each other of carrying out summary executions of prisoners in the city.
In the capital, Syrian troops entered Damascus's southern district of Tadamon with dozens of tanks and armoured vehicles in a push to win back the last rebel stronghold there, a witness and activists said.
Activists said most of the district was under the control of government forces by early Friday evening. The army had been trying to enter Tadamon for more than a week but was pushed back by fierce resistance from the rebels.
An activist said the troops had executed several people after entering the district. The account could not be confirmed.
"Thousands of soldiers have entered the neighbourhood, they are conducting house to house raids," a resident, who did not want to be identified for security reasons, said by telephone.
The fighting spread to Aleppo from Damascus after the bomb attack on Assad's security headquarters in the capital on July 18, which killed four of the president's senior aides and encouraged rebels to step up hostilities.
The Syrian army has reinforced its positions in and around Aleppo over the past two weeks, while conducting daily artillery and aerial bombardments of rebel forces in the city.
"The focus two weeks ago was on Damascus. The focus is now on Aleppo, where there has been a considerable build-up of military means, and where we have reason to believe that the main battle is about to start," Herve Ladsous, the Under-Secretary-General for Peacekeeping Operations, said in New York.
Annan resigned on Thursday, complaining of "finger-pointing" at the United Nations while the bloodshed in Syria went on. His mission, centred on a peace plan and a ceasefire that never took hold, had looked increasingly futile amid escalating violence.
In an article published on the Financial Times website, Annan said Russia, China and Iran "must take concerted efforts to persuade Syria's leadership to change course and embrace a political transition."
"It is clear that President Bashar al-Assad must leave office," Annan said.
However, in a sign that Russia is not yet ready to abandon support for its ally Assad, Moscow hosted a Syrian oil official and promised to send gasoline in return for crude that Syria is having difficulty selling because of sanctions.
"We are ready to deliver all of our oil and receive what we need in gasoline and diesel," Qadri Jamil, Syria's deputy prime minister for economic affairs, told journalists in Moscow.
For both rebels and the government, Aleppo with its 2.5 million inhabitants is a prize that could determine the outcome of a war the opposition says has already cost 18,000 lives.
Internet and telephone networks in Aleppo were mostly cut for the third day, hampering attempts by rebels to coordinate and forcing them to use couriers to deliver orders. Soldiers were launching rockets at insurgents from an infantry school north of Aleppo.
Areas around the city are divided, with some villages loyal to Assad and others favouring the opposition. Police and soldiers were setting up mobile check points on some main roads leading into the city from the north.
There are increasing signs of quarrelling among rebel factions and between fighters and the population.
"The Free Syrian Army is causing us headaches now," said Abu Ahmed, a local official who works with journalists in the Syrian town of Azaz, near the Turkish border.
"If they don't like the actions of a person they tie him up, beat him and arrest him. Personality differences between brigade members are being settled using kidnappings and force. They are self-righteous and we are not happy about it," he told Reuters.
Elsewhere in the country, opposition activists said Syrian forces had killed at least 50 people during clashes with rebels in the central city of Hama on Thursday, while a helicopter bombardment killed 16 rebels near the southern town of Deraa.
In Damascus, at least 20 people were killed on Thursday when security forces fired three mortar rounds at a Palestinian camp that is home to 100,000 refugees, medical sources said. Palestinian leaders in the West Bank and Gaza condemned the attack.