All updates in Universal Coordinated Time (UTC)

19:31 - Two Ukrainian soldiers have been killed in renewed fighting in the eastern city of Slovyansk, according to the AFP news agency.

18:56 - At least 31 people have died in a fire at a trade union building in Odessa's city center, according to Ukraine's interior ministry. The ministry had initially given a higher tally, of 38. The attack appears to have targeted separatists.

18:15 - At least four people have been killed in street clashes between Pro-Moscow and Pro-Kyiv activists in Odessa, according to Reuters news agency, citing local police.

17:20 - Pro-Russian separatists headquarters on fire in Odessa, as shown in this tweet from ABC News Foreign Correspondent Muhammad Lila.

15:20 - Five NATO warships have arrived in the Lithuanian port of Klaipeda to bolster the transatlantic alliance's defense of the Baltic states, according to the AFP news agency. The NATO deployment comes amid rising tensions with Russia over the crisis in Ukraine.

14:24 - US Defense Secretary Chuck Hagel has called on NATO members to increase their defense spending. The US has already deployed additional aircraft and military personnel to NATO allies in Eastern Europe amid aggression from Moscow toward Kyiv.

"We cannot take for granted - even in Europe - that peace is underwritten by the credible deterrent of military power," Hagel said, according to pre-released remarks set to be delivered later on Friday.

14:08 - One protester has been shot dead in Odessa, according to local police. The incident occurred during violent counter rallies between pro-Russian and pro-Kyiv protesters in the Ukrainian port city. Demonstrators were reportedly pelting each other with petrol bombs, cobblestones and other explosive devices.

This video posted by a number of users on Twitter reportedly shows Friday's unrest in Ukraine.

13:21- Ukraine's Foreign Ministry has rejected Russian accusations that Kyiv broke the Geneva peace deal by launching a military operation in Slovyansk. In a press release, the Foreign Ministry accused Moscow of supporting "terrorist groups" in eastern Ukraine. The ministry said that Kyiv was ready to negotiate with Russia to find a solution to the crisis.

The Geneva peace deal - negotiated by the European Union, Russia, Ukraine and the United States - had called for all illegal armed groups to surrender their weapons and to vacate occupied government buildings.

13:17 - Russian Energy Minister Alexander Novak has warned Kyiv that energy giant Gazprom could restrict gas supplies to Ukraine, if the country fails to make prepayments for deliveries by the end of May.

12:40 - Pro-Russian rebels detained three teams of Western journalists for several hours on Friday in eastern Ukraine, according to the AFP news agency. Journalists with US broadcaster CBS News, Britain's Sky News, and the US news website Buzzfeed were detained at checkpoints as they tried to enter the city of Slovyansk.

Clarissa Ward with CBS News said that she was blindfolded and her news team was split up and taken to different locations. A male member of the team was beaten by pro-Russian gunmen, Ward said.The detained journalists have all been released.

11:27 - EU foreign policy chief Catherine Ashton has said it is "vital that we see occupations of buildings reduced," in Ukraine.

10:55 - Russia's Foreign Ministry has called on Western powers to give up their "destructive" policy on Ukraine.

"As we repeatedly warned, the use of the army against one's own people is a crime and is leading Ukraine towards catastrophe," the ministry said in a statement.

10:32 - Speaking on Rossiya 24 television, Kremlin spokesman Dmitry Peskov called on Europe and the United States to assess the situation in Slovyansk where "aviation is being used against the population," according to the report.

He said Kyiv will inevitably be held responsible for making the decision to launch a "punitive operation" in southeast Ukraine, "first of all by its people."

10:00 - German Foreign Minister Frank-Walter Steinmeier has called on pro-Russian rebels to release a group of international observers being held in eastern Ukraine. The Organization for Security and Cooperation in Europe (OSCE) observers have been held since April 25. He made the comments after meeting with OSCE chairman Didier Burkhalter in Switzerland.

09:20 - Pro-Russian separatists have reportedly seized the control center for Donetsk railways in eastern Ukraine. The Reuters news agency quoted a spokesman for the Donetsk railway saying the rebels had taken the command post and that all electricity has been cut. Donetsk lies 110 kilometers (70 miles) south of Slovyansk.

Meanwhile, the DPA news agency has quoted a spokesperson for the pro-Russian activists saying Ukrainian government forces have taken the train station in Slovyansk.

08:38 - Russian President Vladimir Putin's spokesperson has said that Kyiv's "punitive operation" in southeast Ukraine has destroyed the final hope for a Geneva peace deal aimed at de-escalating the crisis.

Peskov also said the Kremlin has sent an envoy to Ukraine's southeast to negotiate the release of the detained OSCE observers.

08:29 - A spokesperson for the separatists, Stella Khorosheva, has told the AP news agency that a pro-Russian activist was killed and another injured in an attack by Ukrainian forces on roadblocks outside Slovyansk.

07:14 - Ukraine's Defense Ministry has said two Mi-24 military helicopters were shot down and their pilots killed, raising the death toll to two.

Authorities say the pro-Moscow separatists used shoulder-launched anti-aircraft missiles to bring down the helicopters.

05:59 - Ukraine's interim Interior Minister Arsen Avakov has confirmed that government forces launched a military operation in Slovyansk just before dawn on Friday. He said a pilot was killed after separatists used anti-aircraft missiles to shoot down a military helicopter in the city.

"Against Ukraine's special forces, terrorists used heavy artillery, including grenade launchers and portable anti-aircraft missile launchers," Avakov posted on his Facebook page. He also warned local residents to stay indoors and keep away from windows.

The operation is the Ukrainian military's first significant response since it launched an "anti-terrorist" operation in early April.

04:07- The self-declared mayor of Slovyansk, Vyacheslav Ponomaryov, was quoted by Russia's Interfax news agency as saying two Ukrainian military helicopters were shot down. He said one pilot was killed and another taken hostage.

03:31 - Reports of a "large-scale operation" by Ukrainian forces to capture the eastern city of Slovyansk are reported by pro-Russian separatists. Heavy gunfire and blasts have been heard on the city's outskirts and a helicopter was seen opening fire.

The city of around 100,000 residents has been a stronghold for pro-Russian separatists who control dozens of government buildings across Ukraine's east. Slovyansk is 160 kilometers (100 miles) west of Russia.

hc/rg (Reuters, AFP, AP, dpa)