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.  QUESTIONS:

Q1:
1. What is Petro Poroshenko?
2. What role has Petro Poroshenko?
3. 
Q2:
1. Petro Poroshenko is president of what country?
2. Which country is Petro Poroshenko president of?
3. 
Q3:
1. What was reported by Ukraine reporters?
2. What was communicated by Ukraine reporters?
3. 
Q4:
1. Which person had called for a ceasefire?
2. What person had called for a ceasefire?
3. 
Q5:
1. Between which countries was the ceasefire called?
2. Between which armies was the ceasefire called?
3. 
Q6:
1. Did everyone agree to the ceasefire?
2. Had everyone committed to the ceasefire?
3. 
Q7:
1. How long a time did it take someone to break the ceasefire?
2. How long after it was called did the ceasefire break?
3. 
Q8:
1. In what place was the agreement to the ceasefire called?
2. In what locality was the agreement about the ceasefire signed?
3. 
Q9:
1. In what country is Minsk located?
2. Where is Minsk located ?
3. 
Q10:
1. Is there trust between the Russian and Ukrainian parties?
2. Can the Russian and Ukrainian parties trust each other?
3. 
Q11:
1. Which town is critical to the ceasefire?
2. What is the name of the town that is becoming critical to the ceasefire?
3. 
Q12:
1. Where is Debaltseve?
2. In what location is Debaltseve?
3. 
Q13:
1. Who rules over Debaltseve?
2. Who has the control over Debaltseve?
3. 
