The regime of dictator Bashar al-Assad struck rebel strongholds hours after the U.S. and Syrian regime ally Russia announced a ceasefire would be implemented on Monday.

Although both the Syrian government and the rebels had reportedly suggested that they would abide by the cessation of hostilities, neither side explicitly vowed to support the deal, which was announced in Geneva on Saturday by Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, reports the Washington Post (WAPO).

CNN reports:

At least 90 people were killed in airstrikes that rained down on northwestern Syria on Saturday and Sunday, a rights group said. The surge in violence came just hours after the US and Russia announced a new ceasefire plan.

The airstrikes landed in the rebel-held areas of Idlib in the northwest and Aleppo in the north of the country, according to the monitoring group Syrian Observatory for Human Rights.

Russia and the United States announced that the ceasefire would begin Monday.

Assad has been emboldened by the increased Russian and Iranian intervention in Syria. He has won many battles and retaken territory with their help.

Hours before the truce went into effect, the Syrian dictator reportedly declared that his government is “determined to recover every area from the terrorists,” echoing similar comments he made earlier this year. Assad considers all opposition forces to be terrorists.

He did not explicitly acknowledge the ceasefire but added that his military would continue its work “without hesitation,” without elaborating further.

“The armed forces are continuing their work, relentlessly and without hesitation, regardless of internal or external circumstances,” he added.

In a letter to the United States, Syrian rebels expressed reservations about the truce but added that they accepted it, in part, due to the humanitarian aid facing their territory.

“Regarding a truce, a ceasefire, the delivery of aid, this is a moral question and there is no debate around this, we absolutely welcome this, but there are other articles around which there are reservations,” Zakaria Malahifji of an Aleppo-based rebel faction told Reuters.

The New York Times (NYT) notes:

Despite widespread pessimism over how long the cease-fire would last, calm was widely reported after it took effect at 7 p.m. local time, with a few notable exceptions.

Less than an hour into the truce, residents in the divided northern city of Aleppo said via text message that a government helicopter had dropped explosive cylinders on a rebel-held district. And in the southern province of Dara’a, a rebel faction said in a statement that it had killed four government soldiers.

NYT notes that many Syrians are possessed by “extensive doubts” about the the ceasefire, which is expected to coincide with the Muslim holiday of Eid-al-Adha would hold.

The Times later explains:

Under the terms, if it holds for seven days, the United States and Russia will undertake a new collaboration of air strikes against jihadist militants in Syria, and the Syrian air force will be barred from flying over insurgent-held areas.

The U.S. supports a Kurdish-led Arab alliance in Syria that is fighting the Islamic State (ISIS/ISIL), and other rebel and Assad forces. Meanwhile, Russia supports Assad, who has been fighting all opposition groups in Syria, including some who are considered allies by the United States.