 -- The United States and Russia have agreed on the terms of another cease-fire in Syria and have called on warring parties across the nation to stop fighting at sundown on Sept. 12, the same day the Muslim Eid al-Adha holiday begins, Secretary of State John Kerry announced from Switzerland tonight.

Kerry said if all sides follow the parameters of the plan, it could be a "turning point” for Syrians and lead to a possible political resolution.

If there is a true cessation of hostilities, Kerry said, Russia and the United States will work together to conduct military strikes against the al-Qaeda-linked Nusra group -- an unlikely alliance between two geopolitical foes.

The Russian and U.S. delegations met for over 14 hours today to finalize the plan.

The plan calls for the Russian backed Syrian regime of Bashar al-Assad to cease all air combat missions over opposition held territory. Kerry said Assad has been masking strikes against U.S. backed rebel forces as legitimate strikes on extremist groups. This new plan appears to map out which groups and where are considered fair game by both parties.

It is estimated that as many as half million people have died inside Syria during five years of civil war.

Despite this new agreement, it remains to be seen whether each side will hold up to its obligations. A previous ceasefire agreed to in February broke down after a matter of weeks. And there remains a great deal of distrust between Russia and the United States. Earlier this week, U.S. Defense Secretary Aston Carter appeared reluctant to work side-by-side with the Russian military, citing its invasion of Ukraine and its aggressive behavior around Europe as just two examples. On Wednesday a Russian fighter jet flew within 10 feet of a U.S Navy reconnaissance plane over the Black Sea — an incident that the Pentagon described as "unsafe and unprofessional."