Mariupol, Ukraine (CNN)Ukraine President Petro Poroshenko declared a ceasefire went into effect at midnight Saturday local time (5 p.m. ET) between the Ukrainian military and pro-Russian militants in the nation's east. 

The separatist Donetsk People's Republic also announced the ceasefire. 

But an apparent violation was reported less than 90 minutes after the ceasefire began when Ukraine's Anti-Terrorist Operation Command reported that a military post near Zolote in the Luhansk region has been shelled by mortars. 

As a positive sign, CNN reporters in Ukraine reported the shelling quieted shortly after midnight. 

The peace agreement signed in Minsk, Belarus, several days ago comes with many questions over how it will be implemented and whether it will stick. Mistrust is high on both sides, adding to the doubts. 

The town of Debaltseve, a strategically located railroad hub in the east, is shaping up as a crucial piece of the ceasefire. 

In his speech announcing the ceasefire, Poroshenko said Ukrainian troops control the town and warned militants to stay away. 

"I think the fact of using the tense situation on the outskirts of Debaltseve by militants-terrorists-mercenaries poses a great threat, which can violate the ceasefire regime," Poroshenko said. "I am warning all participants of the Minsk negotiations, including the Russian Federation, which identifies itself as the guarantor of the reached agreements." 

CNN's Nick Paton Walsh, reporting from the eastern town of Donetsk, said separatists claim to have encircled Debaltseve. 

"Debaltseve will be a vital flashpoint for the hours ahead," he said. 

Poroshenko has warned that if the separatists do not abide by the ceasefire, he will impose martial law throughout Ukraine's territory, his spokesman Andrey Zhigulin told CNN. 
Where was peace agreement signed?
Minsk, Belarus