BEIRUT: Syrian government forces pressed their offensive in a water-rich valley northwest of Damascus Tuesday as 10 rebel groups announced they are suspending talks about planned peace negotiations because of what they described as government violations of a cease-fire deal.

The truce, brokered by Russia and Turkey, is meant to be followed by talks later this month in the Kazakh capital of Astana between mainstream rebel factions and government representatives.

The U.N. Security Council Saturday unanimously adopted a resolution supporting efforts by Russia and Turkey, which back opposing sides in Syria’s civil war, to end the nearly 6-year-old conflict and jump-start peace negotiations.

But the nationwide 4-day-old cease-fire is looking increasingly shaky, with opposition factions angered in particular about the ongoing military offensive in the strategically important Barada Valley.

The government and the opposition disagree about whether the region is part of the cease-fire agreement, which excludes extremist factions such as Daesh (ISIS) and Jabhat Fatah al-Sham, formerly known as the Nusra Front.

The text of the document was never released to the public.

The Syrian government says the mountainous region is not part of the cease-fire because of the presence of Jabhat Fatah al-Sham.

Local activists deny any militant presence in the area. Opposition activists, including the Barada Valley Media Center, Tuesday reported heavy bombardment of villages there. The opposition’s civil defense first responders reported at least nine government airstrikes since Sunday, as well as acute shortages of medical supplies.

Six people have been killed and 73 have been wounded, it said.

In a statement posted late Monday, 10 rebel factions said they were “freezing all discussions regarding the Astana negotiations or any other consultations regarding the cease-fire agreement until it is fully implemented.” They include the powerful Jaish al-Islam, which operates mainly outside Damascus.

The statement said the violations in the Barada Valley are continuing and “threaten the lives of hundreds of thousands of people.”

The statement also said that the opposition will consider any military changes made on the ground to be a serious violation of the cease-fire agreement “that renders it null.”

The Barada Valley, which is controlled by rebels and is surrounded by pro-government forces, including the Lebanese Hezbollah group, is the primary source of water for Damascus and surrounding areas.

The fighting has cut off the capital’s main sources of water, resulting in severe shortages since Dec. 22.

Images from the valley’s Media Center indicate its Ain Fijeh spring and water processing facility have been destroyed, apparently by airstrikes. The government says rebels spoiled the water source with diesel fuel, forcing it to cut supplies to the capital.

The cease-fire agreement, which went into effect early Friday, is supposed to pave the way for the government and the opposition to meet for talks for the first time in nearly a year in the second half of January. The talks will be mediated by Russia, Turkey and Iran, but Russian officials have said other key players including the U.S. are welcome to participate.

In the northern province of Idlib, Jabhat Fatah al-Sham said more than 20 people were killed as a result of U.S.-led coalition airstrikes that targeted one of its command centers. The statement released on the group’s Telegram channel did not give further details, but opposition activists said dozens of people were killed and wounded in the airstrikes that struck the group’s position near the village of Sarmada in the Idlib countryside.

Pentagon spokesman Peter Cook said he was unaware of the incident.

The attack follows an air raid late Sunday that struck several cars traveling on a road leading from Sarmada to the Bab al-Hawa area near the border with Turkey, killing at least eight people, including Al-Qaeda-linked fighters and a senior commander with Chinese Islamist militant faction, according to opposition groups and a local jihadi commander.

The U.S. has killed some of Al-Qaeda’s most senior commanders in Syria over the past two years in airstrikes. Those targeted include members of the so-called Khorasan group, which Washington describes as an internal branch of Al-Qaeda that plans attacks against Western interests.

It was not immediately clear who was behind the Sunday and Tuesday attacks.