Meanwhile, at least 42 people have been killed in street battles between supporters and opponents of Russia in southern Ukraine that ended with dozens of pro-Russian protesters incinerated in a burning building, bringing the country closer to war.
The riot in the Black Sea port of Odessa, ending in a deadly blaze in a besieged trade union building, was by far the worst incident in Ukraine since a February uprising that ended with a pro-Russian president fleeing the country.
It also spread the violence from the eastern separatist heartland to an area far from the Russian frontier, raising the prospect of unrest sweeping more broadly across the country of about 45 million people.
The Kremlin, which has massed tens of thousands of soldiers on Ukraine's eastern border and proclaims the right to invade to protect Russian speakers, said the government in Kiev and its Western backers were responsible for the deaths.
Kiev in turn said the violence was provoked by foreign demonstrators sent in from Transdniestria, a nearby breakaway pro-Russian region of Moldova where Moscow has a military garrison.
Yesterday, Ukraine's security service said illegal military groups from Transdniestria and Russian groups had worked together to foment the Odessa unrest.
"The unrest, which occurred on May 2 in Odessa and led to clashes and many casualties, was due to foreign interference," said a spokeswoman for the SBU security service.
The spokeswoman also accused former top officials, who were once part of ousted Ukrainian president Viktor Yanukovich's inner circle, of financing "saboteurs" to foment the unrest. She named Serhiy Arbuzov and Oleksandr Klymenko.
Klymenko, former incomes minister and a close ally of Yanukovich, denied the allegation and called on Kiev to produce evidence. "I will tell you who is responsible: the blame for bloody Friday in Odessa lies entirely with the current government," he wrote on his Facebook page.
Yesterday morning, people placed flowers near the burned-out doors of the trade union building, lighting candles and putting up the yellow, white and red flag of the city. The burnt remains of a tented camp of pro-Russian demonstrators nearby had been swept away.
About 2000 pro-Russian protesters gathered outside the burnt-out building, chanting: "Odessa is a Russian city."
At the nearby hospital, residents queued up to offer blood and others tried to find out what medicine was needed so they could go and buy it.
Oleg Konstantinov, a journalist covering the events for a localwebsite, said bullets had flown in the melee before the blaze: "I was hit in the arm, then I started crawling, and then got hit in the back and leg," he said.
The Odessa bloodshed came on the same day that Kiev launched its biggest push yet to reassert its control over separatist areas in the east, hundreds of miles away, where armed pro-Russian rebels have proclaimed a "People's Republic of Donetsk".
The rebels there aim to hold a referendum on May 11 on secession from Ukraine, similar to one staged in March in Ukraine's Crimea region, which was seized and annexed by Russia in a move that overturned the post-Cold War diplomatic order.
Yesterday, the government said it was pressing on with the offensive in the area for a second day, and had recaptured a television tower and a security services building from rebels in Kramatorsk, a town near the rebel stronghold of Sloviansk.
"We are not stopping," interior minister Arsen Avakov said in a post on Facebook. "The active phase of the operation continued at dawn."
Rebels in Sloviansk shot down two Ukrainian helicopters on Friday, killing two crew, and stalled an advance by Ukrainian troops in armoured vehicles. Separatists said three fighters and two civilians were killed in Friday's Ukrainian advance on the town.
Vasyl Krutov, head of a government "anti-terrorist centre" behind the operation in the east, told a news conference there was gunfire and fighting around Kramatorsk: "What we are facing in the Donetsk region and in the eastern regions is not just some kind of short-lived uprising, it is in fact a war."
The military operation in the east was overshadowed by the unprecedented violence in Odessa, a vibrant multi-ethnic port city that has seen some support for separatists but nothing like the riots that erupted on Friday.
Police said four people were killed, at least three shot dead, and dozens wounded in running battles between people backing Kiev and pro-Russian activists.
The clashes ended with separatists holed up in the large Soviet-era trade union building. Video footage showed petrol bombs exploding against its walls.
At least 37 people died in the blaze. Yesterday, police raised the overall death toll in the city to 42. It was easily the biggest death toll since about 100 people were killed in Kiev protests that toppled Yanukovich in February.
"Kiev and its Western sponsors are practically provoking the bloodshed and bear direct responsibility for it," Kremlin spokesman Dmitry Peskov was quoted as saying. Peskov also said the violence rendered the idea of holding presidential elections in Ukraine on May 25 "absurd".
Kiev's Interior Ministry blamed the pro-Russian protesters, saying they had attacked the pro-Ukrainians before retreating to the trade union headquarters, from where they opened fire on the crowd and threw out the petrol bombs that caused the blaze.
Odessa is located in the southwest of Ukraine, far from the eastern areas held by the rebels and far from the Russian frontier where Moscow has amassed troops. But it is close to Moldova's Transdniestria region, where Russia also has troops.
The spread of violence to Odessa expands the zone of unrest across the breadth of southern and eastern Ukraine.
"Today we Ukrainians are constantly being pushed into confrontation, into civil conflict, toward the destruction of our country to its heart. We cannot allow this to happen and we must be united in the fight against a foreign enemy," said acting president Oleksandr Turchinov.
Regional police chief Petro Lutsiuk said more than 130 people had been detained and could face charges ranging from rioting to premeditated murder.
The release of the military monitors from the Organisation for Security and Cooperation in Europe (OSCE) resolves a major diplomatic issue for the West.
The separatists had captured the team on April 25 and described them as prisoners of war. One Swede was freed earlier on health grounds while four Germans, a Czech, a Dane and a Pole were still being held until yesterday. A Russian envoy helped negotiate their release.
The separatist leader in Sloviansk, self-proclaimed "people's mayor" Vyacheslav Ponomaryov, said they were freed along with five Ukrainians, with no conditions. "As I promised them, we celebrated my birthday yesterday and they left. As I said, they were my guests," he said.
The OSCE team's leader, German Colonel Axel Schneider, said after being freed: "You can imagine, it's a big relief. The situation was really tough. The last two nights when you see what was going on, every minute gets longer."
Western countries blame Russia for stoking the separatism and fear Moscow could be planning to repeat its annexation of Crimea in other parts of Ukraine.
Russia denies it has such plans, while saying it could intervene if necessary to protect Russian speakers, a new doctrine unveiled by President Vladimir Putin in March that overturned decades of post-Soviet diplomacy.
On Friday, Western leaders said they would seek tougher measures, including hitting whole sectors of the Russian economy, if Moscow interferes with Ukraine's May 25 vote.