Ukraine's army is starting the withdrawal of heavy weapons from the front line in the east as part of a truce, the defence ministry says.
It says the pullout of 100mm calibre artillery "is the first step" and will be monitored by foreign observers.
Pro-Russian rebels earlier said they had started withdrawing their weapons. This has not been verified by monitors.
The ceasefire came into effect on 15 February but the rebels seized the key town of Debaltseve just days later.
Fighting began in Ukraine's eastern Donetsk and Luhansk regions last April, a month after Russia annexed the Crimea peninsula.
Almost 5,800 people have died since then, the UN has estimated, although it believes the real figure could be considerably higher.
The Ukrainian government, Western leaders and Nato say there is clear evidence that Russia is helping the rebels with heavy weapons and soldiers.
Independent experts echo that accusation while Moscow denies it, insisting that any Russians serving with the rebels are "volunteers".
In other developments on Thursday:
- Russia warned it might have to cut off gas supplies to Ukraine by the end of the week if it did not receive further payments from Kiev. Ukraine said fewer than half of its prepaid shipments had been piped from Russia
- The EU urged Russia "to urgently release" Ukrainian military pilot Nadiya Savchenko on humanitarian grounds. Ms Savchenko - who is accused of involvement in the deaths of two Russian journalists - has spent 75 days on hunger strike in detention in Moscow
In a statement, the Ukrainian defence ministry said Thursday's weapons pullout was part of the ceasefire agreed in Minsk, Belarus, on 12 February.
It warned that if there were any "attempts to attack", the pullout timetable would be "corrected".
The decision to begin moving artillery from the front line came shortly after Ukraine's military said its forces had suffered no fatalities in the past 48 hours, although several soldiers had been wounded.
The separatist rebels in the self-proclaimed Donetsk People's Republic and Luhansk People's Republic say they have been pulling out their heavy weapons for the past several days.
The rebels say the process is being monitored by international observers from the Organization for Security and Co-operation in Europe (OSCE).
Analysis: David Stern, BBC News, Kiev
At first glance, Kiev's announcement that its forces would start to withdraw heavy weapons from the front line is positive news - and possibly a sign that the two sides are finally carrying out the Minsk agreement.
But making sure the deal sticks will remain a day-to-day struggle. Shelling continues, and the Ukrainians have said they reserve the right to "change the withdrawal schedule accordingly" should fighting resume.
We will also have to wait to see if the OSCE monitors give their blessing to the Ukrainians' effort. So far, they have held off on giving their approval to the rebels' own withdrawal, saying the insurgents have restricted the observers' movements and provided scant information.
We will wait to see if the withdrawal actually begins and, if it does, whether it is carried out fully by both sides. Still, the simple fact that the bloodletting has been greatly reduced - if only for the moment - is welcome.
In a statement on Wednesday, the OSCE special monitoring mission said it had observed movement of trucks and howitzers in several rebel-held areas.
However, OSCE Parliamentary Assembly President Ilkka Kanerva later said he was "profoundly disturbed by the illegal separatists' continuing refusal to grant unlimited, safe access to OSCE monitors on the ground in Ukraine and their violations of the Minsk Package of Measures".
"By extension, I am also disturbed by Russia's apparent unwillingness to publicly insist that the rebels immediately allow the monitors access, stop all ceasefire violations and withdraw all heavy weapons."
Under terms agreed in Minsk, both Ukraine's government forces and the rebels must pull out their heavy weapons, creating a buffer zone of at least 50km (30 miles).
The process is meant to be monitored by OSCE observers.