Eighteen civilians were killed on Saturday when Syrian government jets launched strikes in the region of Idlib - the country's last opposition stronghold - despite a cease-fire there agreed by Turkey and Russia.
Seven were killed in the city of Idlib, the capital of the province of the same name, seven in the town of Binish and four in the village of Nairab, the Syrian Observatory for Human Rights said.
Dozens were also injured in the strikes, observatory head Rami Abdel-Rahman told dpa.
The reported attacks came two days after the Russian military announced that a cease-fire has taken effect in Idlib in accordance with agreements with Turkey. On Friday, Ankara said the truce would start Saturday at midnight.
Russian President Vladimir Putin and his Turkish counterpart, Recep Tayyip Erdogan, expressed support for the cease-fire as they met in Istanbul earlier this week.
Moscow is an ally of Syrian President Bashar al-Assad while Ankara supports the Syrian opposition.
Idlib, located in north-western Syria, is the last rebel stronghold in the country.
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Government forces, assisted by the Russian military, began an offensive in the region last year. The move displaced hundreds of thousands of people, compounding a humanitarian crisis in Syria that nearly nine years.
Nine aid groups on Saturday criticized the United Nations Security Council after it had voted to extend the delivery of humanitarian assistance to Syria through two border crossings, while halting humanitarian access through the country's north-east near the Iraqi border due to Russian pressure.
"This is another unacceptable example of countries putting politics above people and will ultimately result in further unnecessary suffering," the groups said in a statement.
"In north-east Syria, where the cross-border lifeline used by the United Nations has been closed thanks to [Friday's] vote, an estimated 1,650,000 people are reliant on humanitarian assistance," they added.
Moscow has argued that cross-border assistance is no longer needed because it is being delivered by the Syrian government, which does not approve of the UN aid mechanism.
According to UN sources, the new UN Security Council decision re-authorized two crossings of Bab al-Salam and Bab al-Hawa that connect Syria with Turkey.
The Friday resolution dropped re-authorization of the crossing of al-Ramtha on the Syrian-Jordanian border as well as the crossing of Yaribieh between Syria and Iraq, the sources added.
David Swanson, a regional spokesman for the UN Office for the Coordination of Humanitarian Affairs (OCHA) for Syria, said the "horrific humanitarian situation" in north-western and north-eastern Syria underscores the critical role that cross-border operations play, providing vital life-saving assistance to millions of people.
"There is no immediate alternative to reach the millions of people in need at the scale provided by cross-border operations," he told dpa.
"Significantly, the need for cross-border deliveries is not expected to reduce in the coming months," he added.