Syrian troops are shelling the rebel-held neighbourhood of Khaldiyeh in the central city of Homs, according to activists.
The activists said the bombardment appears to be a preparation for the troops to storm the area, which has been in rebel hands for months.
The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees had no immediate word on casualties.
Amateur videos posted online showed a small white plane, apparently a drone, flying over Homs.
The city has been one of the hardest hit regions in Syria since an uprising against president Bashar Assad's regime began in March last year.
Today's violence comes two days after reports of mass killing in the nearby province of Hama, where about 80 people, including women and children, were shot, stabbed or burned.
UN observers came under fire yesterday as they tried to reach the site of the reported killings in Mazraat al-Qubair, a small farming community of 160 people, mostly Bedouins.
In Geneva, International Committee of the Red Cross spokesman Hicham Hassan told reporters today that the humanitarian situation in Syria was worsening.
"Currently the situation is extremely tense, not only in Houla, not only in Hama, but in many, many places around the country," he said.
The string of villages known as Houla is where more than 100 people were massacred last month. The Opposition and the regime blamed each other for that massacre.
Hassan cited the countryside of the northern city of Idlib, suburbs of the capital Damascus, the eastern province of Deir el-Zour and the coastal region of Latakia as those targeted in the latest attacks
The ICRC wants to help 1.5 million people, some of whom need basic assistance such as the provision of bread.
Hassan said many are also worried about people they have left behind, adding that most of those who fled from Taldaw, a village in the Houla region, were women and children.
"They don't know what happened to the people who remained," he said.
The opposition has also called for anti-government protests today after the weekly noon prayers.
It was still not clear if observers have entered Mazraat al-Qubair, where activists said dozens of people, including women and children, were killed on Wednesday.
Activists said the Sunni village is surrounded by Alawite villages. Alawites are an offshoot of Shiite Islam and Assad is a member of the sect, while the opposition is dominated by Sunnis.
A government statement aired yesterday on the state-run news agency SANA said "an armed terrorist group committed an appalling crime" in Mazraat al-Qubair, killing nine women and children.
It said residents appealed for protection from Hama authorities, who sent security forces who went to the farm, stormed a hideout of the group and clashed with its fighters.
The United States also condemned Assad, saying he has "doubled down on his brutality and duplicity".
International envoy Kofi Annan, whose peace plan brokered in April has not been implemented, warned against allowing "mass killings to become part of everyday reality in Syria".
"If things do not change, the future is likely to be one of brutal repression, massacres, sectarian violence, and even all-out civil war," Annan told the UN General Assembly in New York. "All Syrians will lose."
UN diplomats said Annan was proposing that world powers and key regional players, including Iran, come up with a new strategy to end the 15-month conflict at a closed meeting of the Security Council that took place yesterday.