- "The danger of full-scale civil war is ... real," U.N. chief says
- Diplomats echo a sense of urgency in ending bloodshed after reports of mass killings
- The killings spark more outrage against President Bashar al-Assad
- Opposition activists report more shelling Friday
(CNN) -- International envoy Kofi Annan meets U.S. Secretary of State Hillary Clinton on Friday as frustrated world leaders warned that Syria is engaging in crimes against humanity and drifting closer to a civil war.
"How many more times have we to condemn them and how many ways must we say we are outraged?" U.N. Secretary-General Ban Ki-moon said Thursday after 10 hours of talks on Syria. "The danger of full-scale civil war is imminent and real, with catastrophic consequences for Syria and the region."
Diplomats echoed a sense of urgency in ending escalating bloodshed after reports of another massacre that left dozens dead this week.
"Enough is enough," Arab League chief Nabil el-Araby said as he called for "all kinds of pressure" on Damascus.
Annan's meeting with Clinton in Washington comes a day after he briefed the United Nations in talks that emphasized the need to halt the attacks.
The latest mass killings sparked outrage against President Bashar al-Assad, whose regime is accused of targeting civilians to suppress a 15-month uprising against his government.
A delegation denied entry into Syria last week interviewed witnesses in neighboring countries who described torture, threats and attacks, according to a top U.N. official.
"There is unselective shelling, there is deliberate targeting with live munition of protesters, there is systematic torture going on in prisons," said Ivan Simonovic, the U.N. assistant secretary-general for human rights.
Simonovic said descriptions included threats and torture involving burning and putting people in unnatural positions.
"It's appalling," he told CNN's Christiane Amanpour.
Hours after his assessment, opposition activists reported heavy shelling in Homs on Friday and an explosion at a police station in Idlib that caused an unknown number of casualties among regime forces.
Annan, who is serving as a mediator on Syria for the United Nations and the Arab League, said the situation is dire.
"You could say we are drifting, if we are not already in, a sort of a civil war," Annan said. "All efforts are being made that, if it were to become a full-blown civil war, it doesn't spread to neighbors."
In April, Annan brokered a peace plan in Syria that included calls for warring sides to silence their guns and lay down their weapons. But provisions in the plan have not been met, including the release of arbitrarily detained people.
Though some detainees have been freed, thousands remain in custody and the whereabouts of others are unclear, according to the United Nations.
Top envoys at the United Nations Security Council meeting on Thursday highlighted the urgency of the situation on the ground.
Ban noted there is minimal evidence that the regime is complying with its commitment under the Annan plan, saying the opposition "is hardening and turning increasingly to arms."
Killings in recent weeks may amount to crimes against humanity, according to the U.N.
"The confrontations in certain areas of the country have taken on the character of an internal conflict subject to international humanitarian law and possible war crimes prosecution," Ban said.
U.N. monitors in the nation are not safe, either.
They came under fire Thursday as they tried to reach Qubeir, the site of the latest reported mass killing that opposition activists said left 78 people dead, including women and children. Soldiers blocked observers heading to the village to verify reports of the Wednesday killings, the U.N. said, but they plan to make more attempts.
Another mass killing in Houla two weeks ago left more than 100 people dead.
Syrian has reiterated that anti-government terrorists, and not the regime, are responsible for the bloodshed.
CNN cannot independently confirm reports from within Syria because the government limits access by international journalists.
The United Nations has said that more than 9,000 people have died in Syria since anti-government protests started in March 2011. But death counts from opposition groups range from more than 12,000 to more than 14,000. Tens of thousands of people have been displaced.
A planned June 20 summit in Los Cabos, Mexico, will include discussions on how to proceed, Ban said.
CNN's Saad Abedine contributed to this report.
