Syrian President Bashar al-Assad has said that the army's battle with rebel forces would determine the fate of his country, and praised soldiers for confronting what he said were "criminal terrorist gangs".
"The fate of our people and our nation, past, present and future, depends on this battle," Assad, who has not spoken in public for two weeks, said in a written statement marking Armed Forces Day on Wednesday.
Assad said the army was waging a "heroic" battle against the enemy and that the country was engaged in a "crucial battle for its destiny".
Earlier on Wednesday, fierce fighting between the rebels and the army erupted near two Christian areas of central Damascus for the first time in the nearly 17-month uprising, as the battle for Aleppo continued into its 12th day.
Also on Wednesday, the United Nations mission in Syria said its observers had witnessed government fighter jets opening fire on Aleppo, the country's largest city.
In a briefing on Wednesday, mission spokeswoman Sausan Ghosheh also said the UN had confirmation that the rebels now have heavy weapons of their own, including tanks.
Ghosheh expressed concern over the situation in Aleppo, describing "heavy use of heavy weapons, including tanks, helicopters, heavy machine guns, as well as artillery".
In fighting across other parts of the country, at least one government soldier was killed in clashes in Damascus' Bab Touma and Bab Sharqi districts, the Syrian Observatory for Human Rights, a UK-based activist group, said.
"This is fighting in areas where it has not happened before. These are areas where the rebels have so far not had access," Observatory chief Rami Abdel Rahman told the AFP news agency.
Bab Touma and Bab Sharqi in the Old City of Damascus were previously popular with tourists and were the scene of several pro-regime protests in the past.
Furthermore, the Local Co-ordination Committees (LCC), which organises protests on the ground, reported a blast and heavy gunfire from Baghdad Street, a main route in Syria's capital.
The LCC also said the capital's southern suburb of Tadamon was hit by mortar fire at dawn.
'Mother of battles'
Since July 20, the battle between the rebels and government forces has focused on Aleppo.
There were bombings during the night to Wednesday northwest of Aleppo, while heavy machinegun fire and rocket fire were also reported in the area.
In-depth coverage of escalating violence across Syria
"The army and the terrorist groups have both sent reinforcements for a decisive battle that should last several weeks" in Aleppo, a Damascus security source told the AFP.
The rebels sent in reinforcements from nearby Turkey after seizing a strategic checkpoint just outside Aleppo.
"The Syrian army is surrounding rebel districts, and is bombing them, but it is going to take its time before it launches its assault on each neighbourhood," the security source said.
The Observatory said fighting on Tuesday in Aleppo was the fiercest so far in a contest that state media had billed as the "mother of all battles".
"Hundreds of rebels attacked the police stations in Salhin and Bab al-Nayrab [neighbourhoods] and at least 40 policemen were killed during the fighting, which lasted for hours," Abdel Rahman said.
They later seized a third police station in Hanano, the Observatory said.
Amnesty International, the human rights group, accused regime forces of conducting a brutal crackdown against dissidents in Aleppo.
"The current onslaught on the city of Aleppo, which puts civilians even more at grave risk, is a predictable development which follows the disturbing pattern of abuses by state forces across the country," said Amnesty's Donatella Rovera in a report on the Syrian crisis.
The International Committee of the Red Cross and the UNHCR have reported that 200,000 people have fled the Aleppo area amid "continuous raging violence".