The agreement to swap 125 Ukrainian servicemen for 225 rebels held by Kiev followed peace talks between envoys of Ukraine, Russia, the separatists and European security watchdog OSCE on Wednesday.
"We've prepared 225 people, which we will hand over. The main thing is for this not to fall through now," the SBU's Markiyan Lubkyvsky told Reuters.
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.
The pro-Western government in Kiev accuses Russia of orchestrating the rebellion in Ukraine's east, a charge denied by Moscow.
Exchanging prisoners is one of the criteria of a 12-point peace protocol, which also includes a ceasefire, agreed by Kiev and rebels in September. But most of the plan has not yet been implemented due to repeated violations of the ceasefire and because separatists defied Kiev by holding leadership elections.
Ukraine's Interfax news agency quoted a rebel representative as saying the prisoners would be swapped by Dec. 30. Lubkyvsky did not confirm this date, but said the exchange would happen "soon".
It is not known exactly how many prisoners are currently held by the two sides, but Ukraine's military said this month around 600 Ukrainians were in rebel hands.
Around 1,300 people have been killed since the ceasefire was agreed in September, according to the United Nations, but the fighting has lessened significantly in December.
On Friday, however, the military said rebels had slightly stepped up their attacks on Ukrainian positions in the east of the country and reported that one Ukrainian servicemen had been killed in 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.
Kiev authorities had said there could be further peace talks between Ukraine, Russia, the separatists and the OSCE in Minsk on Friday, but Belarus's Foreign Minister Dmitry Mironchik said they would not take place, Belarussian Interfax reported, without elaborating.
