After two false starts, the Ukrainian army now appears to be pressing on with this offensive designed to restore Kiev's control over Donetsk. But Vasyl Krutov, the head of Ukraine's "anti-terrorist" command, cautioned that the situation was more serious than had been thought. "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."
Gunfire and clashes were still happening around Kramatorsk, Krutov added. Ukrainian forces had managed to recapture the local headquarters of the SBU intelligence service in Kramatorsk, the Interior Ministry said later, and the television tower.
But Kiev believes that Russian Spetsnaz special forces soldiers, aided by operatives from the GRU - or military intelligence - have organised the seizure of public buildings in a dozen towns and cities across Donetsk and the neighbouring region of Luhansk. The pro-Russian insurgents have achieved complete control over Slaviansk, located near large arms dumps dating from the Soviet era that contain as many as 2.5 million Kalashnikov assault rifles.
Ukrainian forces have tried to seal off Slaviansk by capturing pro-Russian checkpoints on its perimeter. On Saturday, this effort cost them two military helicopters, both of which appeared to have been shot down with surface-to-air missiles, strengthening the evidence that Russia has been supplying the gunmen.
So far, Ukraine's army has not attempted a full-scale assault to retake the town. All of its decisions are taken under the shadow of the possible response from Vladimir Putin, the Russian President. Russia has massed between 35,000 and 40,000 troops on Ukraine's eastern frontier, stockpiling enough fuel, ammunition and medical supplies - including a chain of field hospitals - for this army to be able to mount an invasion within 12 hours of receiving the order.
Watch: Pro-Russian protesters burned in deadly fire in Odessa
The incidents on Saturday, particularly in Odessa where 31 pro-Russian protesters were burnt to death inside a public building, could provide the pretext for a Russian advance into eastern Ukraine. The Kremlin could argue that its soldiers are needed to restore peace and to protect ethnic Russians. In Donetsk, the pro-Moscow leaders of a self-proclaimed "People's Republic" have already urged the deployment of Russian troops as "peacekeepers".
Yesterday, they voiced outrage over the bloodshed in Odessa. Denis Pushilin, the self-styled prime minister, said it was unimaginable that such killings could have occurred on "our land, on Russian land". Dmitry Peskov, the Kremlin's spokesman, blamed Ukraine's new rulers yesterday, saying they were "up to their elbows in blood".
Ominously, he added that Russia had been inundated with thousands of pleas for help from inside Ukraine. Russia had not yet decided how to respond, he added, pointing to the unprecedented scale of the recent bloodshed.
"This element is absolutely new to us," said Peskov, according to the Interfax news agency. "Kiev and its Western sponsors are practically provoking the bloodshed and bear direct responsibility for it."
John Kerry, the US Secretary of State, spoke to his Russian counterpart, Sergei Lavrov, by telephone yesterday. Kerry said that Russia must "withdraw support for the separatists" and begin to "de-escalate the situation". Lavrov, by contrast, said the burden of restoring calm rested on America. He urged Washington to restrain the Kiev Government and, in particular, force a halt to the military offensive in the east.
Russia appeared to have used its influence to secure the release of seven military observers from the Organisation for Security and Co-operation in Europe. The army officers - four from Germany and one each from Denmark, Poland and the Czech Republic - were freed alongside five Ukrainians who had been acting as their escort. They had spent eight days in the hands of the insurgents.
Vyacheslav Ponomarev, the pro-Russian "people's mayor" of Slaviansk, denied that outside pressure had played a part in securing their release, saying the observers had been set free in accordance with his own earlier promise.
Colonel Axel Schneider, a German officer who was among the detainees, told AP that all 12 were in good spirits. "They had a very good attitude and that gave them the strength to stand the situation. According to the word of [Ponomarev], we have been treated as good as possible. This is a miserable situation, but we were under his protection."
The release of the OSCE team appears to have been brokered by Vladimir Lukin, a Russian envoy in eastern Ukraine, who visited Slaviansk yesterday. He said the observers had been allowed to leave as a "humanitarian" gesture.
