Ukraine's military says it will be "forced to return artillery and mortars" to its eastern front line if fighting in the country's east escalates further.
Military spokesman Andriy Lysenko made the comments on November 16 as fighting between government forces and Russian-backed separatists intensified after weeks of relative calm.
Ukraine says six of its soldiers have been killed around the rebel stronghold of Donetsk over the weekend despite a September cease-fire agreement.
"I have given orders to open fire in response as soon as our troops' lives come under threat," President Petro Poroshenko warned on November 15.
Meanwhile, separatists in Donetsk said Kyiv breached the truce dozens of time over the past week.
More than 7,900 people have been killed in fighting in eastern Ukraine since April 2014.