Syria's city of Homs has been subjected to its most severe bombardment in five months, activists say.
Aircraft and artillery targeted the neighbourhood of Khaldiya, the Syrian Observatory for Human Rights said.
Activists also reported fierce clashes in the second city Aleppo, and government shelling in the capital Damascus, Hama and Idlib.
Turkey has meanwhile reinforced its border following a deadly Syrian mortar strike on a Turkish town.
Prime Minister Recep Tayyip Erdogan warned Syria at a large rally in Istanbul on Friday that Turkey would defend its sovereignty.
"We are not interested in war, but we're not far from it either," he said.
Syrian government forces had subjected Homs to months of intense bombardment until April, after which the focus of the violence shifted to other cities.
But on Friday activists reported military strikes on Khaldiya, and also on the districts of Old Homs, Qusour and Jourat al-Shayah.
The head of the UK-based activist group the Syrian Observatory for Human Rights, Rami Abdel Rahman, told AFP news agency: "It seems like the regime has a limited window to use its warplanes, because it is throwing everything it can at the rebels in Homs."
One Homs-based activist, Abu Rami, told Associated Press: "Around dawn, the regime went crazy and started shelling hysterically. An average of five rockets a minute are falling."
The Observatory also said there had been renewed shelling of rebel positions in the neighbourhood of Sakhour in Aleppo, which state TV described as a "cleansing of terrorists and mercenaries".
The Local Co-ordination Committees, an opposition activist network, reported anti-government demonstrations on Friday in a number of towns and cities, including Damascus and Aleppo.
Meanwhile, rebel fighters said they had captured an air base with a stock of missiles outside the capital, Damascus.
Rebels released video showing smoke rising from the installation in the eastern Ghouta area, along with captured missiles, but the footage has not been independently verified. The rebels have increasingly targeted air bases as government forces have stepped up the use of air strikes.
Later, activists posted videos online which they said showed a military aircraft being shot down by rebel fighters as it bombarded towns in eastern Ghouta. It was unclear if it was a helicopter or fighter jet.
According to activists, more than 30,000 people have been killed since the uprising against President Bashar al-Assad began in March last year. The UN estimates that at least 20,000 have died.
The Observatory put Thursday's death toll from fighting in Syria at 180.
The group is one of the most prominent organisations documenting and reporting incidents and casualties in the Syrian conflict. The group says its reports are impartial, though its information cannot be independently verified."
Military build up
On Thursday, the UN Security Council said the Syrian mortar strike on the Turkish border town of Akcakale - which is believed to have been an accident - underscored the grave impact the Syrian crisis was having on "regional peace and stability". Two women and three children were killed.
Turkey responded by shelling Syrian army positions.
On Friday the Security Council also condemned what it called "terrorist attacks" in Aleppo on Wednesday that killed dozens of Syrian civilians. The council said the al-Nusra Front, affiliated with al-Qaeda, had said it carried out the attacks.
Following the Akcakale deaths, Turkey's parliament authorised troops to launch cross-border operations against Syria for a period of one year.
On Friday, Turkey moved tanks and anti-aircraft missiles into Akcakale, although Mr Erdogan has said his country does not intend to start a war with Syria.
At the Istanbul rally he warned: "Those who attempt to test Turkey's deterrence, its decisiveness, its capacity, I say here they are making a fatal mistake."
A Turkish foreign ministry official told AP Syria had pulled tanks and other materiel away from the border.
There were no reports of fresh cross-border clashes, although the situation remains tense.
One resident of Oncul, close to Akcakale, told AP: "Our store owners, our citizens and our children are all very concerned. We did not sleep until morning."