The reported pullback was in line with a new trust-building agreement that was signed on September 1 since when only seven Ukrainian servicemen have been killed.
"Today, we have completed the arms withdrawal," Donetsk ground commander Eduard Basurin told AFP by telephone.
He said the last nine mortars had been pulled back from Ilovaisk -- a frontline town where Ukraine lost more that 100 servicemen in a rebel onslaught in August.
The pullback was witnessed by observers from the Organisation for Security and Co-operation in Europe (OSCE).
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A similar deal regarding larger weapons that was struck during international negotiations in February was never fully implemented.
And over the past few days, AFP teams have heard repeated gun battles being waged after dark around the separatists' de facto capital Donetsk.
OSCE monitors say they have been barred by rebels from accessing the area and have seen arms that had been pulled back later returned to their original positions.
Kiev announced the end of a similar pullback from the smaller pro-Russian Lugansk region on Tuesday.
The proposed buffer zone is meant to stretch 30-kilometres wide and be overseen by OSCE monitors from both Russia and the West.
The September 1 deal was unexpectedly signed after a series of broken ceasefires that had left world leaders scrambling to find a way to halt a conflict which has killed more than 8,000 people in 18 months.
It was negotiated without the presence of the rebels' political backer Russia or Germany and France -- the main European sponsors of previous peace talks and close allies of Ukrainian President Petro Poroshenko.