* Exchange is part of 12-point peace plan
* Ukraine says rebels using ceasefire to regroup (Adds OSCE comments on peace talks)
By Pavel Polityuk and Alessandra Prentice
KIEV, Dec 26 (Reuters) - Ukrainian authorities and separatists exchanged hundreds of prisoners of war on Friday, a spokesman for President Petro Poroshenko said, part of a 12-point plan aimed at ending the pro-Russian uprising in the east of the country.
The agreement to swap 150 Ukrainian servicemen for 222 rebels followed peace talks between envoys of Ukraine, Russia, the separatists and European security watchdog Organization for Security and Co-operation in Europe (OSCE) on Wednesday.
"The head of the SBU (security service) reported the release of 146 Ukrainians to the President. The SBU expects another four prisoners to be released tomorrow. They will all be able to celebrate New Year ... with their families," spokesman Svyatoslav Tsegolko said in a Facebook post.
Earlier, an SBU aide had said they would hand over 225 rebels for the Ukrainian servicemen.
The uprising by separatists began a month after Russia annexed the Black Sea peninsula of Crimea from Ukraine in March, following the popular overthrow of Ukraine's Moscow-backed president. The conflict has killed more than 4,700 people.
Kiev's pro-Western government says Russia orchestrated the rebellion in Ukraine's east, a charge denied by Moscow.
The peace protocol, agreed by Kiev and rebels in September, also includes a ceasefire. Most of the plan has not been implemented due to repeated violations of the ceasefire and because separatists defied Kiev by holding leadership elections.
It is not known exactly how many prisoners are held by the two sides, but Ukraine's military said this month about 600 Ukrainians were in rebel hands.
Roughly 1,300 people have been killed since the ceasefire was agreed in September, according to the United Nations, but the fighting lessened significantly in December.
On Friday, however, the military said rebels had slightly stepped up their attacks on Ukrainian positions in the east and reported that a Ukrainian servicemen had been killed during the past 24 hours.
"In the past two days, (rebel) fighters started using artillery and GRAD rocket launchers. Attacks have intensified to a minor extent," military spokesman Andriy Lysenko told Reuters.
"Rebels are using the ceasefire to regroup their forces," he said later in a televised briefing.
Further peace talks between Ukraine, Russia, the separatists and the OSCE had been expected to take place again in Minsk on Friday, but OSCE Chairperson-in-Office, Didier Burkhalter, said in a statement the meeting had not taken place and expressed the hope it would be rescheduled as soon as possible.
Burkhalter said the purpose of the talks was to make further progress in four key areas of the Minsk protocol - the implementation of the ceasefire, the release of hostages, the delivery of humanitarian aid and some economic issues. (Editing by Louise Ireland)