(CNN) The proposed "cessation of hostilities" in Syria, announced jointly earlier this week by Russia and the United States, went into effect at midnight Friday in Damascus (5 p.m. ET).

U.N. Special Envoy Staffan De Mistura said at a press briefing in Geneva that immediate reports indicated "suddenly both Daraa and Damascus had calmed down."

An unusual quiet descended on major cities in Syria, said Rami Abdulrahman, director of the London-based Syrian Observatory for Human Rights, adding there were brief exceptions including some unexplained explosions reported in the north.

Nobody expects peace to settle over the nation overnight, De Mistura said.

"There are some high chances that we should expect hiccups," he said.

If there are violations, he said, "the important point that we need to see is that if those incidents will be quickly brought under control and contained.

'Our best chance'

Another Syria task force meeting will take place Saturday afternoon in Geneva to assess whether the temporary truce is being respected, he said.

The cessation of hostilities is the most hopeful sign in years in the nation that's been torn by civil war. At least 250,000 people have died and at least 1 million people have fled the nation.

"The full implementation of this resolution -- including unimpeded and sustainable humanitarian deliveries -- is our best chance to reduce the brutal violence in Syria. What matters now are not the words of the resolution but whether it will make real changes on the ground and reduce the suffering of the Syrian people," U.N. Secretary General Ban Ki-moon said in a written statement.

Earlier Friday, the Security Council voted unanimously in favor of a resolution backing the halt in fighting and asking for all parties to abide by the terms of the cessation.

Meanwhile, a main Syrian opposition group said it will respect the two-week truce that starts with the cessation of hostilities but it warned the government and Russia not to target it under a pretense of attacking internationally recognized terror groups like ISIS.

The rebel High Negotiations Committee said Friday that 97 factions agreed to abide by the deal, but it warned the Syrian regime and one of its major foreign supporters, Russia, not to attack the factions under a pretense of targeting internationally recognized terror groups in the region, such as ISIS and the al Qaeda-linked al-Nusra Front.

Syria has been riven by years of seemingly intractable war.

"It should not be possible for the regime and its allies to exploit the ... truce by continuing hostilities against moderate opposition factions under the excuse of fighting terrorism," the panel said in a published statement Friday.

The two-week truce does not apply to "terrorist organizations designated by the U.N. Security Council," including ISIS and al-Nusra. Military operations against them are expected to continue.

This is an important detail because there are more than 160 armed factions on the battlefield.

Meanwhile, United Nations Special Envoy to Syria Staffan De Mistura said Friday he planned to resume the intra-Syrian peace talks on Monday, assuming the cessation of hostilities holds. Included will be representatives of the Syrian government and opposition groups, he said.

The agreement

The agreement calls for the Syrian regime and the opposition fighters to halt attacks and implement a U.N. "road map" for peace.

Under the terms, Russia, whose warplanes have targeted non-ISIS and non-al Qaeda Syrian opposition groups, will halt those airstrikes.

Both sides agree to allow humanitarian agencies access to the territories they hold, and to refrain from taking territory held by the other side.

The "road map" is U.N. Security Council Resolution 2254, which the council adopted unanimously in December.

As Friday's deadline approached, regime and Russian airstrikes continued to pound different parts of the country.

On Friday, Russian President Vladimir Putin said the truce would be a difficult and "contradictive" process.

"But there's no alternative to a peaceful settlement of the conflict," Putin told a meeting of members of the Russian federal security service, the FSB. "All conditions have to be created for the soonest end of the bloodshed and for an inter-Syrian political dialogue in the future with participation of all constructive political forces."

Civil war has raged over Syria for five years, since protests during the hopeful days of the Arab Spring were brutally repressed.

More than a quarter of a million people have died so far. Half the country's population has been uprooted and has fled. People in some Syrian cities are starving. More than a million people entered Europe without the required papers last year -- most of them, by far, Syrians. The European Union's commitment to the free, borderless movement of people is in danger of collapse.