The United Nations says "mass atrocities" continue to take place in Syria as Islamic extremists grow more powerful and foreign fighters enter the war.
A new U.N. commission report says the impact of the war is "no longer confined to Syrian territory."
Commission chairman Paulo Pinheiro said Islamic State militants have taken over large parts of eastern Syria and northern Iraq. The report says the Islamic State's gains in Iraq have boosted its military capabilities, and the militants are increasingly fighting other rebel groups for power in Syria.
The commission noted the international support meant to boost "moderate" opposition fighters, saying those efforts to provide money and equipment have not helped reverse the dominance of radical armed groups operating in Syria.
The report also calls attention to the lack of an enduring international push to find a resolution to the crisis, saying "influential states have turned away from the difficult work required for a political solution."
Long-sought face-to-face peace talks mediated by former U.N. envoy for Syria Lakhdar Brahimi concluded earlier this year with little progress.
Pinheiro said the fighting in Syria is continuing "with no regard to law or to conscience" as hundreds of people die every day.
Death toll in Syria Civil War, 2011 - 2014
The U.N. said last week that more than 191,000 people have been killed since the conflict began in March 2011.
Wednesday's report says the apparent goal of the Syrian military's operations is "to render life unbearable in areas out of its control," detailing the use of sieges followed by bombing campaigns and the arrests of men who are of fighting age.
It also says the government has used illegal chlorine gas in its attacks, and is "systematically targeting civilians and civilian infrastructure."
The commission recommends that the U.N. Security Council consider referring the crisis to the International Criminal Court, and calls on all parties to stop abuses that include murder, rape, torture and forced disappearances.
Meanwhile, Syrian rebels from at least two different groups seized control of a border crossing with Israel's Golan Heights Wednesday, killing 20 Syrian. Israel's military said the fighting spilled over into Israeli territory, wounding an Israeli officer.
U.S. journalist Peter Theo Curtis, who was held hostage by Syrian militants for two years, spoke Wednesday to reporters, a day after coming home to the United States. He expressed surprise at the amount of media attention given his case, and gratitude for the effort put into securing his release.
The mother of another U.S. hostage is pleading with captives to free her son. Shirley Sotloff says her son Steven should not pay for U.S. government policies in the Middle East.
She appealed to the leader of Islamic State to follow in the tradition of the Prophet Muhammad by showing mercy and protecting the lives of all Muslims, Christians and Jews.
Islamic State, which beheaded American journalist James Foley earlier this month, is threatening to kill Sotloff if the U.S. does not stop airstrikes on Islamic State targets in Iraq.