A flag used by the Syrian opposition is unfurled Wednesday amid rubble in the Damascus neighborhood of Jobar. (Amer Almohibany/AFP/Getty Images)

Syria’s partial cease-fire showed signs of wobbling Friday amid mounting accusations of government assaults and a lack of promised humanitarian aid deliveries to thousands of besieged people.

Groups opposed to Syrian President Bashar al-Assad issued blistering warnings, saying that ongoing government offensives threaten to throttle the nearly week-old truce and the proposed resumption of U.N. peace talks Wednesday.

But the apparent attacks — by Assad’s forces and their Russian allies — also point to the gray areas that raised questions about the cease-fire even before it began. Under the provisions, military strikes are still permitted against some groups, including the Islamic State and an al-Qaeda-linked faction that has battled Assad’s troops.

That gives potential room for airstrikes and ground offensives spilling over against other rebel groups that oppose Assad but that so far say they are willing to abide by the cease-fire.

Rebel fighters in multiple areas of the war-torn country reported heavy assaults by pro-government forces that included suspected airstrikes by Russia, which intervened militarily last year to prop up Assad.

The United States, Russia and other powers came to an agreement on a ‘cessation of hostilities’ in Syria, but the deal was met with caution and skepticism. (Jason Aldag,Ishaan Tharoor/The Washington Post)

“We’re getting bombarded by 50 airstrikes a day by the Russians,” said Abu al-Nour, a rebel who is battling a pro-government offensive in the mountains of the northwestern Latakia province near the border with Turkey. He used a nom de guerre out of concern for his safety.

In echoes of the Friday demonstrations that marked the initially peaceful 2011 uprising against Assad, Syrians in rebel-held areas held rallies, chanting slogans common during the Arab Spring revolts. Images and videos posted on social media showed people taking to the streets in areas besieged for years by government forces.

The agreement to reduce hostilities took effect last weekend as a result of strong backing by Russia and the United States, which support opposing sides in a war that has killed more than 250,000 people and uprooted millions. The countries co-chair a task force that adjudicates reported violations.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, reported that 118 people, including 24 civilians, have been killed in areas covered by the cease-fire during the first five days. The agreement excludes groups that are classified as terrorists, such as the Islamic State and al-Qaeda-affiliated Jabhat al-Nusra.

The U.N. envoy to Syria, Staffan de Mistura, acknowledged violations in remarks Thursday, but he said in general that the cease-fire “has been holding” and has greatly reduced violence.

De Mistura’s comments appeared starkly at odds with a statement released later by the powerful rebel group Jaish al-Islam.

The group said the cease-fire had failed to stop pro-government advances, including near the rebel group’s stronghold east of Damascus, the capital.

The war, the group said in its statement, “effectively did not stop on the ground” and has continued near the capital and in flash points such as Homs and Aleppo.

The High Negotiations Committee, a Saudi-backed opposition coalition, also issued statements accusing Assad and his allies of land-grab attempts and attacks against civilians.

The coalition’s chief coordinator, Riyad Hijab, said during an address in Paris on Friday that it was “too soon” for the opposition to decide whether it would participate in the U.N.-backed talks.

He questioned how Russia could be an arbiter of the truce while also active in the conflict. He also pointed out that another key provision to the pact — the rapid delivery of humanitarian aid to Syrians living under siege — had been stalled because of obstruction by the Syrian government.

Hundreds of thousands of Syrians are at risk of starvation because of sieges in multiple locations across the country by both opposition groups and pro-government forces. The majority of those people are besieged by the government, which has been accused of systematically violating the laws of war by denying people access to food and medicine.

“Aid convoys are stopped at regime checkpoints for long periods on the excuse of searching them under the eyes of the world, and Syrians are still dying daily from hunger and lack of supplies,” said a High Negotiations Committee’s statement.

The opposition group has promised to observe the cease-fire for two weeks.

U.N. officials and aid workers blame the Syrian government for the obstruction of new deliveries of humanitarian aid. “It’s clear that the government has been giving us the back-and-forth in all of this,” said one official, speaking on the condition of anonymity because of a lack of authorization to discuss the issue.

Some aid has reached one stricken area since last Saturday, the government-besieged Moadamiya suburb of Damascus. But the official said the government prevented some deliveries of medical equipment from reaching the suburb, where residents and aid workers warn of deteriorating humanitarian conditions.

The United Nations is overseeing aid deliveries during the cease-fire. On Sunday, the U.N. resident coordinator in Damascus, Yacoub El Hillo, said 154,000 people living in besieged areas would receive food and medical aid in five days.

“It is the best opportunity that the Syrian people have had over the last five years for lasting peace and stability,” Hillo said Sunday, referring to the cease-fire.

Zakaria Zakaria in Istanbul contributed to this report.