Syrian government forces pressed their offensive in a water-rich valley northwest of Damascus on 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 United Nations Security Council on Saturday unanimously adopted a resolution supporting efforts by Russia and Turkey, which back opposing sides in Syria's civil war, to end the nearly six-year conflict and jump-start peace negotiations.

But the nationwide four-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 the Islamic State group and al-Qaida's affiliate, known as the Fatah al-Sham Front. The text of the document was never released to the public.

The Syrian government said the mountainous region is not part of the cease-fire because of the presence of the Fatah al-Sham Front. Local activists deny any militant presence in the area.

Heavy bombardment

Opposition activists, including the Barada Valley Media Center, on 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 on 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 Army of Islam group, which operates mainly outside the Syrian capital.

It 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 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 al-Fijeh spring and water processing facility have been destroyed. The government said 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 on 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. Those talks will be mediated by Russia, Turkey and Iran, though Russian officials have said other key players, including the United States, are welcome to participate.