Damascus and Syria's second biggest city, Aleppo, came under shell fire from government forces battling a growing insurgency against President Bashar al-Assad on Thursday, opposition activists in the area said.
Residents in southern Damascus reported a shell landing in southern areas of the capital every minute.
After a major assault on rebels in Damascus last week, the army has turned to Aleppo, reinforcing troops there with an armoured column that had been operating in a northern province.
Fierce clashes raged in the early hours in Aleppo, and an activist said rebels now controlled half of Syria's commercial capital, a claim that could not be independently verified.
"There was shelling this morning on the Salaheddine and Mashhad districts," said Aleppo activist Abu Hisham. "Now it stopped, but helicopters are buzzing overhead."
Activists said 24 people were killed in fighting in and around Aleppo on Wednesday, swelling a national death toll of about 18,000 since the revolt against Mr. Assad began 16 months ago.
In the Syrian capital, a resident in the Yarmouk Palestinian refugee camp reported heavy shelling, particularly near the southern Hajar al-Aswad district.
She said the army seemed to be targeting sites on the edges of the camp, firing shells every minute. The bombardment started around 7 a.m. local time and was still going three hours later.
As hostilities have intensified in the north, in and around Aleppo, Turkey closed its border posts to commercial traffic on Wednesday, but not to refugees fleeing Syria.
The revolt against Mr. Assad has developed from an insurgency in the provinces into a battle for control of the two main cities, Aleppo and Damascus, where fighting exploded last week.
Mr. Assad's forces have launched big counter-assaults in both cities. They appear to have beaten rebels back from neighbourhoods in the capital and are attacking Aleppo.
At the Syrian town of Azaz, a few kilometres south of the Turkish border, rebels appeared in control after heavy clashes over the past month in which they succeeded in driving government forces out of what had become a rubble-strewn ghost town.
The conflict has left Azaz in ruins, a Reuters correspondent who visited the town said. Burned-out armoured personnel carriers sat on the roads where rebels had hit them with rocket-propelled grenades. Bullet casings were scattered everywhere.
Most residents fled during the latest fighting, which pushed Assad's forces out over the past month and ended in the rebels taking the Bab al-Salam border crossing with Turkey on Sunday.
The struggle for Aleppo could prompt an exodus across the Turkish border, where some Syrian refugees are already complaining about poor conditions in camps and have clashed with riot police in disputes over food and other issues this week.
"There is not enough food. They have broken our hearts, the Turks. Why are they doing this to us?" said a sobbing woman called Umm Omar, with her four children huddled next to her in a camp near the border.
In its most recent comment on the fighting, state-run Syrian television said on Wednesday that government troops were imposing security and stability in and around Aleppo.
"The terrorists are suffering terrible losses. Groups of them are throwing their weapons away and giving themselves up. Others are fleeing for the Turkish border," the television said.
Military experts believe an overstretched Syrian army is pulling back to concentrate on fighting insurgents in Aleppo and Damascus, important power centres for the government, while leaving outlying areas in the hands of rebels.
The uprising has entered a more violent phase in the past 10 days since rebels poured into Damascus in large numbers.
On July 18, an explosion killed four members of Mr. Assad's inner circle inside a security headquarters, a blow that wiped out much of the top echelon of his military command structure and shattered the reputation for invulnerability that his family has held since his father seized power in a coup in 1970.
Western powers have been calling for Mr. Assad to be removed from office for months, but they fear that he will fight to the end, raising the risk of sectarian warfare spreading across one of the world's most volatile regions.
UN Secretary-General Ban Ki-moon, addressing the Bosnian parliament in Sarajevo, said the world must unite to end the "slaughter" in Syria, recalling the inertia of the United Nations in 1995 during the Srebrenica massacre in Bosnia.
At the UN Security Council, members blamed each other for rising violence in Syria. Western states pledged to seek an end to the conflict outside the world body, while Russia warned of "likely catastrophic consequences" with that approach.
Russia, an ally of Syria, and China have repeatedly blocked Western-backed Security Council resolutions on Syria.
As the revolt against Mr. Assad intensifies, more Syrian officials have abandoned their posts. Syria's ambassador to the United Arab Emirates and his wife, who is Damascus's envoy to Cyprus, defected on Wednesday.
A military attache at the Syrian embassy in Oman told Al Jazeera television he had also defected. "After the killing and bombardment and destruction of our people in Syria increased, and the massacres took place, this ... pushed me to defect from this regime," Mohammad Tahseen al-Faqir said.