This ambiguity has radicalized the conflict, raised the political stakes and contributed to civilian suffering, said Paulo Pinheiro, the chairman of the U.N.-backed Independent Syria Commission group in an interview Monday.

“We have said this to the states themselves. We have said it’s better to be fully committed to the political process instead,” said Mr. Pinheiro. “The airspace [above Syria] is overcrowded and it has humanitarian consequences.”

The 31-page report, which laid out a detailed account of a nation at the brink of collapse, is the 11th produced since the commission was formed in 2011 to investigate and document Syria’s war. The report offers a list of recommendations for a lasting peace to Syria’s government, the opposition and the international and regional powers involved directly or through proxy groups. Most of what it has so far recommended has fallen on deaf ears.

Mr. Pinheiro said the Syrian government has categorically rejected the commission’s recommendations, such as ensuring humanitarian access to besieged areas and ending the targeting of civilian areas. The opposition, too, has ignored calls to abide by international humanitarian laws, he said.

The report also blames “international and regional powers”—meaning the U.S., Russia, France and U.K., as well as Saudi Arabia and Iran—for sustaining the conflict through military assistance and for failing to adequately use their leverage with the warring parties to strike a peace settlement.

One recommendation that Mr. Pinheiro said has been ignored by the international community is a request that countries stop supplying weapons to warring factions in Syria.

“Paradoxically, the international and regional stakeholders that are ostensibly pushing for a peaceful solution to the war are the same that continue to feed the military escalation,” the report said.

The U.N. Security Council has been at the forefront of mapping a political plan for Syria, but at the same time four of its five veto-holding members are active parties to the conflict, a situation that one diplomat said has no precedent since the Korean War in the 1950s.

One on side of the conflict is Russia, which heeded a call from Syria’s government for military assistant and is bombing terrorist and opposition outposts. On the other side are the U.S., U.K. and France, which have offered training, funding and weapons to opposition armed groups and have carried out airstrikes on Islamist terrorist groups like Islamic State and Nusra Front.

China, the fifth veto holding power at the Council, has refrained from direct involvement in the conflict, but Syrian opposition members say Chinese companies have assisted Syria’s regime with surveillance and cyber technology to crack down on the opposition.

A Security Council diplomat said that while the countries’ involvement complicates peace negotiations, their vested interests could serve as a common cause to rally behind. So far, he said, that common cause has yielded resolutions that allow members to take military action against terrorist targets inside Syria and one that lays out an ambitious plan for a political transition and peace. None, however, has had any traction on the ground in Syria.

Mr. Pinheiro said the top three obstacles to Syria’s peace were the multiplication of war fronts, the factions connected and supported by outside countries and the growing internationalization of the war.

The Security Council, Mr. Pinheiro said, could play a pivotal role as the only international body with the power to implement binding resolutions and redirect the Council’s attention from military engagement in Syria to a political process.

Secretary of State John Kerry said on Monday that the only means to reach a peace deal in Syria would be through a Security Council resolution approved by all members, including Russia.

“The outcome we’ve gotten is to have everybody who’s a stakeholder at the same table, all of them agreeing in this process,” said Mr. Kerry, “in order to get a U.N. Security Council resolution outlining a framework for a political settlement, and Russia voting for it.”

The U.N. report also concludes that there have been flagrant violations of human-rights law and rampant war crimes on both sides of the conflict. Mr. Pinheiro said the most disturbing were recurring attacks on schools and hospitals targeting children and the sick, the most vulnerable of the civilian population.

“These are war crimes. We say to both sides: just stop attacking civilians,” he said.