The United States and Russia have agreed on a new cease-fire for Syria that will take effect Saturday, even as major questions over enforcing and responding to violations of the truce were left unresolved. Syria's warring government and rebels still need to accept the deal.

The timeline for a hoped-for breakthrough comes after the former Cold War foes, backing opposing sides in the conflict, said they finalized the details of a "cessation of hostilities" between President Bashar Assad's government and armed opposition groups after five years of violence that has killed more than 250,000 people.

The truce will not cover the Islamic State group, the al-Qaida-linked Nusra Front and any other militias designated as terrorist organizations by the U.N. Security Council. But where in Syria the fighting must stop and where counterterrorism operations can continue must still be addressed. And the five-page plan released by the U.S. State Department leaves open how breaches of the cease-fire will be identified or punished.

The announcement came after Presidents Barack Obama and Vladimir Putin spoke by telephone Monday, capping weeks of intense diplomacy to stem the violence so that Assad's government and "moderate" rebel forces might return to peace talks in Geneva. A first round of indirect discussions collapsed almost immediately this month amid a massive government offensive backed by Russian airstrikes in the northern Syria.

Obama welcomed the agreement in the call with Putin that the White House said was arranged at the Russian's request. The White House said Obama emphasized the key is to ensure that Syria's government and opposition groups faithfully implement the deal.

"This is going to be difficult to implement," said White House spokesman Josh Earnest. "We know there are a lot of obstacles, and there are sure to be some setbacks."

Putin called the agreement a "real step forward that can stop the bloodshed." Speaking on Russian television, he said Moscow would work with the Syrian government, and expects Washington to do the same with the opposition groups that it supports.

U.N. Secretary-General Ban Ki-moon also welcomed the agreement, saying it is "a long-awaited signal of hope to the Syrian people." But he warned that much work lies ahead for its implementation.

The leader of a Saudi-backed Syrian opposition alliance said in a statement that rebel factions have agreed "in principle" to an internationally mediated temporary truce. Riad Hijab did not elaborate but urged Russia, Iran and the Assad government to end attacks, lift blockades and release prisoners held in Syria.

Syrian officials said the government was ready to take part in a truce as long as it is not used by militants to reinforce their positions.

Both sides have until Friday to formally accept the plan.

Even if the cease-fire takes hold, fighting will by no means halt.

Russia will surely press on with an air campaign that it insists is targeting terrorists but which the U.S. and its partners say is mainly killing moderate rebels and civilians. While IS tries to expand its self-proclaimed caliphate in Syria and neighboring Iraq, Nusra is unlikely to end its effort to overthrow Assad. The Kurds have been fighting IS, even as they face attacks from America's NATO ally Turkey. And Assad has his own history of broken promises when it comes to military action.

All of these dynamics make the truce hard to maintain.

"We are all aware of the significant challenges ahead," U.S. Secretary of State John Kerry said. "Over the coming days, we will be working to secure commitments from key parties that they will abide by the terms."

Kerry said the cessation could lead to less violence, expanded humanitarian deliveries and help support the U.S. goal of a "political transition to a government that is responsive to the desires of the Syrian people." Like previous U.S.-Russian statements, however, Monday's document says nothing about Assad's future — perhaps the biggest stumbling block to a sustained peace.

The plan largely follows the blueprint set by Washington, Moscow and 15 other countries at a conference in Germany earlier this month. That agreement called for a truce by Feb. 19, a deadline that was missed.

Beyond the new cease-fire date, the agreement sets up a "communications hotline" and, if needed, a working group to promote and monitor the truce. Violations are to be addressed by the working group with an eye toward restoring compliance and cooling tensions. The deal also calls for "non-forcible means" to be exhausted before other means are pursued for punishing transgressors.

Any party can report violations to the working group being co-chaired by the U.S. and Russia.

The two countries also will share "pertinent information" about territory held by rebels accepting the truce.

The timing of the cease-fire is only days ahead of Moscow's proposal earlier this month for it to start on March 1. Washington rejected that offer at the time, saying it wanted an "immediate cease-fire" and not one that would allow Syria and its Russian backer to make a last-ditch effort for territorial gains in the Arab country's north and south.

While negotiations dragged, however, Russian airstrikes pummeled areas in and around Aleppo, Syria's largest city, and Assad's military made significant gains.

Inside the Hamidiyeh Souk, a popular Damascus bazaar, people expressed worries that a cease-fire wouldn't be evenly observed, leaving Syrian authorities vulnerable.

"I hope there will be no cease-fire, because if there is a cease-fire, Turks will increase their support for criminals and traitors," said Ahmad al-Omar, who is from Aleppo province in the north, adding that Turkey may let opposition fighters in through its border with Syria. The Associated Press reported from the bazaar on a government-approved visit.

But rebels who engage in violence could see their Western support cut off. In recent days, U.S. officials have spoken about the cease-fire being a "self-policing" mechanism. If a group fights Assad's military, according to this logic, it essentially aligns itself with militants considered to be terrorists and can then be attacked.

That has the opposition concerned about Assad or Russia trying to provoke it into acts of self-defense.

The conflict began with violent government repression of largely peaceful protests in 2011 but quickly became a full-blown rebellion against Assad and a proxy battle between his Shiite-backed government and Sunni-supported rebels. A U.S.-led coalition is only attacking the Islamic State and other extremist groups, not Assad's military.

The Syrian government's supply route by land to the city of Aleppo was cut by heavy fighting Monday as the army, supported by allied militias and the Russian air force, fought to consolidate its recent gains. They are trying to seal the border with Turkey, a key supporter of the rebels, before a truce is reached.