Ukrainian President Petro Poroshenko warned on Thursday he was considering proclaiming martial law as the conflict with pro-Russian separatists has escalated in the eastern parts of his country.
It was not clear whether martial law, if declared, would be limited to some parts or the whole of the country.
"If the situation escalates in the east and in Crimea we don't rule out the possibilities [that] we will be forced to introduce martial law and announce a [further] mobilization," Poroshenko said in a televised speech on Thursday from the western region of Lviv.
According to the Ukrainian constitution the president can impose martial through a presidential decree, but this needs approval from the parliament.
If martial law is declared it would restrict the movement of people in Ukraine and ban public gatherings of political parties.
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Poroshenko made his comments as fresh tension with Russia over Crimea reignited fears that a fragile ceasefire deal hammered out in the Belarussian capital Minsk in February 2015 could collapse following the deadliest month of fighting in a year.
Moscow and Kiev have been trading allegations against each other. Russia has accused Ukraine of being behind alleged attacks on its soldiers in Crimea, with Ukraine rejecting the allegations and holding Russia responsible for a fresh wave of attacks against its forces.
On Thursday the Ukrainian military reported that three servicemen had been killed and six wounded in fighting over the past 24 hours. Kiev claimed Russian-backed separatists were behind the deaths of its servicemen.
PHOTO: Fighting in Eastern Ukraine
-by Ukrainian military photographer Dmitriy Muravskiy. pic.twitter.com/h8iDvXQsQW
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Meanwhile, separatist officials accused the Ukrainian military of firing at rebel-held territory, including with heavy weapons that were meant to be withdrawn from the front line according to the Minsk deal, the separatist news site DAN reported.
Since 2014, Ukraine's pro-Western leadership has announced several waves of mobilisation, calling up Ukrainian men to join the military in its fight in the eastern Donbass region.
The Organization for Security and Co-operation in Europe, which is responsible for monitoring the implementation of the 18-month-old ceasefire agreement, has reported a "high number" of violations in eastern regions this week. That includes artillery and gunfire and the movement of heavy weapons.