Syria's air force killed at least 20 people, including 17 children, on Wednesday in the latest strike on battered Aleppo city, as civilians bear the brunt of the civil war.
The air strikes came a day after at least 100 people, including some 80 civilians, died in a twin car bomb attack on a pro-regime area of Homs claimed by the Al-Qaeda affiliated Al-Nusra Front.
Despite the ongoing conflict, the regime of Bashar al-Assad is preparing for a presidential election slammed as a farce by the opposition and by the United States as a "parody of democracy".
In Aleppo, Syria's second city, twin air strikes hit a school in the rebel-held Ansari neighbourhood Wednesday, the Syrian Observatory for Human Rights said.
Amateur video distributed by activists showed rows of bodies of children, some of them bloodied, wrapped in grey body bags on the ground.
Aleppo-based citizen journalist Mohammed al-Khatieb told AFP by Internet that the children were "holding a drawing exhibition when two air strikes, 10 minutes apart, struck the school."
Aleppo province's opposition council condemned the attack in a statement, and appealed to the "world's conscience" for action over Syria, "for the sake of the values and principles you defend, because your silence is killing us".
Rebel-held areas of Aleppo have come under massive assault from the air since mid-December.
The government's use of barrel bombs -- unguided munitions usually dropped from altitude to avoid ground fire -- has come in for particular criticism from human rights watchdogs because of their indiscriminate toll on civilians.
Hours later, Al-Nusra Front carried out two suicide attacks in the Aleppo countryside, killing and wounding "dozens of troops and pro-regime militiamen", said the Observatory.
Al-Nusra Front had earlier claimed an attack on Tuesday against Homs' Abbasiyeh neighbourhood, mainly inhabited by members of Assad's Alawite minority community.
"God allowed the Al-Nusra Front's fighters to achieve a feat despite draconian security measures. It is so that they (residents of pro-regime areas) taste the hell that our brothers have tasted," a statement said.
According to Observatory director Rami Abdel Rahman, the attacks come amid a major government offensive aimed at reclaiming a handful of rebel areas in the heart of Homs.
Hundreds of people have been killed and thousands of families displaced from Homs by violence since the anti-Assad revolt broke out in March 2011.
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The attacks come just weeks away from a presidential election that will be held in regime-controlled territory.
According to Abdel Rahman, the escalation in attacks is a message from the armed opposition to the regime to say there will be no safe zones during the election.
Meanwhile, the number of presidential hopefuls shot up to 17, all of whom except Assad are unknown to the Syrian public.
The election has been mocked as a "farce" mainly because of a series of constraints that make it effectively impossible for a genuine regime opponent to run.
'Aid not working'
Meanwhile, another massive camp was established in neighbouring Jordan to offer shelter to tens of thousands of refugees.
Azraq camp "is probably the biggest refugee camp in the world," said UNHCR representative Andrew Harper.
Jordan has taken in nearly 600,000 refugees since the war broke out.
In New York, UN humanitarian chief Valerie Amos said a Security Council resolution aimed at ensuring aid access to millions of desperate Syrians is "not working."
"Far from getting better the situation is getting worse," Amos said.
Less than 10 percent of 242,000 people living in besieged areas had received assistance in the past four weeks, she said.
Meanwhile, British Foreign Secretary William Hague pressed the world's chemical weapons watchdog to finish its probe into the alleged use of chlorine gas in Syria as urgently as possible.
The Organisation for the Prohibition of Chemical Weapons and the United Nations are already destroying Syria's chemical weapons as part of a disarmament deal agreed last August in the wake of deadly sarin nerve agent attacks outside Damascus.
France and the United States first highlighted the allegations this month that Assad's regime was using the chemical chlorine against civilians despite the deal.