An internationally brokered cease-fire went into effect early Saturday in Syria amid widespread doubts that the truce would halt the fighting or bring the fractured nation closer to peace after almost five years of war.
Fierce fighting was reported in several areas of the country as various factions appeared to be seeking to maximize their gains before the cease-fire began.
Samantha Power, the U.S. ambassador to the United Nation, said Friday that making the cease-fire hold was "a very big if."
Still, a major opposition bloc, the High Negotiations Committee, said Friday that it was committed to the limited truce for two weeks. The group, which is based in Saudi Arabia, says it represents almost 100 opposition factions.
The Syrian government of President Bashar Assad and its principal backers, Russia and Iran, have also vowed to abide by the terms of the truce, which was brokered by the U.S. and Russia and endorsed Friday in a vote by the U.N. Security Council.
But the agreement does not include Islamic State or the Nusra Front, both of which have been deemed terrorist organizations by the U.N.
Both the United States and Russia are expected to continue bombing campaigns targeting those groups. On Friday, the leader of the Nusra Front called for intensified attacks against pro-government forces in Syria.
The last major peace effort in Syria came in 2012, when the war was still in its early stages and Kofi Annan was the U.N. special envoy for the country. The Nobel laureate and former U.N. secretary-general resigned his post, labeling the assignment "mission impossible."
While Russia and the U.S. sit on opposite sides of the conflict, both seek to stop the threat of Islamic radicalism emanating from Syria and alleviate the humanitarian catastrophe inside the country, where more than 200,000 have been killed and entire neighborhoods have been turned to rubble.
European nations besieged by war refugees also have a strong interest in ending the conflict.
But any effort to pause the fighting faces long odds.
The multi-sided conflict features unusual alliances of convenience and odd bedfellows, with hundreds of factions of varying political, sectarian, ethnic and regional affiliations fighting on different sides.
Russia intervened on behalf of Assad's government nearly five months ago and launched an air onslaught that has helped turn the tide of battle. Assad's forces are closing in on opposition-held eastern Aleppo, raising the prospect that the city could be reunited under government control for the first time in almost four years.
In recent weeks, fighting in the north has sent tens of thousands of Syrians fleeing to the border with Turkey, which has provided aid camps on its side of the frontier but blocked most from going into the interior. Turkey already houses more than 2 million Syrian refugees.
Opposition groups involved in the cease-fire have voiced concern that Russian warplanes will continue targeting them as allies of the Nusra Front. The Al Qaeda affiliate operates in concert with many opposition militias dubbed "moderate" by Washington.
"We fear that Russia will use the deal to target the moderate factions in Syria," Salem al Meslet, spokesman for the High Negotiations Committee opposition coalition, said this week.
Russia, which views Assad as a close ally and bulwark against Islamic extremists, has said it has no intention of ceasing attacks against "terrorists," which is how Assad refers to all armed rebels.
Even for those following the war closely, tracking the numerous players and alliances can be difficult.
Armed factions backed by the U.S. regularly cooperate with forces linked to Al Qaeda. Rebels routinely face off against pro-government Shiite militias trained by Iran. The Shiite Muslim theocracy in Tehran is a major supporter of Assad's secular government.
Hard-line Sunni Islamist factions dominate the Syrian armed opposition. However, much of the Syrian army arrayed against them is composed of Sunni Muslim conscripts whose two-year service terms have been extended indefinitely.
Washington's major allies against Islamic State militants are fighters affiliated with the Syrian offshoot of the Kurdistan Workers' Party, which has been waging a war against Turkey for more than three decades and is regarded as a terrorist organization by both the U.S. and Turkey.
Turkey, a U.S. ally, has regularly shelled Syrian Kurdish forces who receive U.S. assistance. The Syrian Kurds, in turn, have fought with Syrian rebel groups that have received U.S. aid.
Turkey, which has demanded that Assad resign, has warned that the cease-fire is "not binding" if Ankara's security is threatened. Turkey, along with Saudi Arabia and Qatar, have been major backers of sundry rebel factions in Syria.
Russia and Turkey have been at loggerheads since Ankara shot down a Russian jet last November in the skies above the Syrian-Turkish border. Russian-backed Syrian forces have moved to seal the border, choking off supplies to Turkish proxies fighting inside Syria.
The cease-fire will be monitored by a task force co-chaired by Russia and the U.S., which will face the daunting task of delineating territory held by Islamic State, Nusra Front and other "terrorist organizations."
Staffan de Mistura, the U.N. special envoy to Syria, told the Security Council on Friday that the cease-fire marked a "crossroads."
"It is potentially, a historic junction — to bring an end to the killing and destruction and to start a new life and new hope for the Syrians," he said.
Both the U.S. and Russia hold that the only long-term solution to the Syrian crisis is a broad political deal to form a representative transitional government in Damascus and lead to the adoption of a new Syrian constitution and U.N.-backed elections.
"None of us are under any illusions," President Obama said this week about the cease-fire. "We're all aware of the many potential pitfalls, and there are plenty of reasons for skepticism, but history would judge us harshly if we did not do our part in at least trying to end this terrible conflict with diplomacy."