Ukraine's army said on Wednesday that a new nine-day truce with pro-Russian insurgents was holdings despite small skirmishes that claimed the lives of two government soldiers this week.

"A relative calm on the front is holding," Ukrainian military spokesman Andriy Lysenko told reporters.

Ukrainian President Petro Poroshenko said Saturday that the Western-backed truce signed in Minsk in February had been respected this week for the first time, despite pro-Russian rebels claiming a civilian had been killed.

"I have some good news. There has been no shooting on the front for a week," Poroshenko said in televised remarks. "This is the first week that the Minsk agreements are being implemented."

Two of the U.S. Air Force's most advanced planes, F-22 Raptor stealth fighters, made a landmark visit to Estonia Friday, the first time the state-of-the-art aircraft have visited former Soviet territory.

The visit comes as tensions are running high with neighboring Russia over its role in the conflict in Ukraine.

NATO chief Jens Stoltenberg said Thursday a renewed ceasefire in eastern Ukraine and Kiev’s decentralization reforms created a new opportunity for peace and asked Moscow to stop backing the rebels.

"I welcome the efforts to have a renewed ceasefire," Stoltenberg said, referring to an agreement between the government and separatists to end ceasefire violations from Tuesday as children went back to school.

Ukraine's security services said Thursday they had detained four members of a pro-Russian group suspected of plotting to blow up a block of flats in Kiev.

"They planned to blow up a building in Obolon and were caught red-handed," Olena Gitlyanska, spokeswoman for the SBU security services, told AFP, referring to a large residential area in northern Kiev.

The United States is co-hosting drills in the Black Sea with what is left of Ukraine's devastated navy, which lost about two thirds of its sailors and ships after Russia seized Crimea last year.

The maritime operation, dubbed Sea Breeze, began Tuesday and is an annual exercise that this year includes contingents from 11 countries practicing air defense, search-and-rescue procedures and a variety of other tactical maneuvers aimed at boosting regional security.

Two civilians were killed Wednesday in eastern Ukraine, undermining fresh efforts to end continuing violence as the European Union extended sanctions against individuals deemed responsible for the conflict.

A Western-brokered ceasefire agreed in February has been punctuated by frequent deadly incidents.

NATO on Tuesday activated command units in six central and eastern European members to boost defense amid regional tensions over Russia's actions in Ukraine, Lithuania's defense ministry said.

"NATO today officially activated NATO force integration units in Lithuania, Latvia, Estonia, Poland, Bulgaria and Romania," ministry spokeswoman Asta Galdikaite told AFP.

Ukraine was in shock Tuesday after fierce clashes outside parliament killed three policemen and wounded 140 people as tensions flared over controversial legislation giving more autonomy to pro-Russian rebels.

It was the worst unrest in Kiev since a bloody uprising ousted a Moscow-backed president in early 2014, unleashing a separatist insurgency in the industrial east that has killed thousands of people.

One policeman was killed and dozens injured Monday in street battles with protesters in Kiev as Ukrainian lawmakers gave their initial backing to controversial legislation granting more autonomy to pro-Kremlin rebel regions.

It was the worst unrest in the capital since a bloody popular uprising ousted Moscow-backed Viktor Yanukovych early last year, an event that set in train the separatist insurgency in Ukraine's industrial east.