The latest deaths came as the new UN envoy to the country acknowledged brokering an end to the nation's civil war would be a "very, very difficult" task.
The British-based Syrian Observatory for Human Rights and the Local Coordination Committees (LCCs) said an area in the northern town of al Bab, about 20 miles from the Turkish border, was targeted.
The Observatory said 18 died; the LCCs put the death toll at 25.
The Syrian uprising against President Bashar al Assad's regime has morphed into a civil war in the face of a brutal Government crackdown. Activists say at least 23,000 people have been killed.
The violence has escalated in recent months, and activists said on Sunday that some 5000 people were killed in August alone – the highest reported in more than 17 months of bloodshed.
Yesterday, activists reported violence across the country, including the suburbs of the capital Damascus, the eastern region of Deir el Zour, as well as in Daraa to the south and in Idlib and Aleppo to the north. Activists and state media said a roadside bomb in the Damascus suburb of Jaramana wounded several. Diplomatic efforts, including a six-point peace plan by former UN Secretary-General Kofi Annan, have failed so far. Now Lakhdar Brahimi has taken over his peace envoy role.
He admitted: "It is definitely a very, very difficult mission."
In Damascus, Information Minister Omran al Zoebi vowed Syria "will give Brahimi every possible assistance. We will give him maximum assistance the way we did with Kofi Annan."
Mr al Zoebi also sought to deflect some of the responsibility for the success or failure of Brahimi's mission to Turkey, Saudi Arabia and Qatar, saying they must "stop sending weapons (to rebels) and close training bases," they are hosting.
Turkey, Saudi Arabia and Qatar are among the harshest critics of Mr Assad's regime and strongly support the rebels.
France's foreign minister has said Western powers are preparing a tough response if the Syrian President's regime deploys chemical or biological weapons in its civil war. Laurent Fabius said "our response ... would be massive and blistering".
Meanwhile, US president Barack Obama has said that if the regime was to use chemical or biological weapons, this would be a "red line" issue.
Mr Fabius said Russia and China are "of the same position", but acknowledged frustration at their support for Mr Assad.
Syria's leadership has said it could use chemical or biological weapons if it was attacked from outside.
It is believed to have nerve agents as well as mustard gas and Scud missiles capable of delivering lethal chemicals.
The International Committee of the Red Cross (ICRC) says its new president is heading to Syria on a three-day visit – his first since he took office.
Peter Maurer was due to meet Mr Assad and several of his ministers.
The Red Cross said the talks will address the "rapidly deteriorating humanitarian situation" and the difficulties which the ICRC and the Syrian Arab Red Crescent face in reaching people affected by Syria's civil war.
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