A deadly rocket attack on the port city of Mariupol on Saturday highlighted a week-long rebel advance that has seen the separatists take the long-disputed Donetsk airport and pledge to conquer more territory.
A major offensive in eastern Ukraine by rebels allegedly backed by Russia has marked a dangerous new stage in the nine-month conflict and dashed hopes for a truce, pressuring Western nations to act.
Rebels did not claim responsibility for the Grad and Uragan rockets that killed 30 people, but monitors from the Organization for Security and Co-operation in Europe said they were fired from the direction of separatist-held areas.
Mariupol is the last major city in the country's two separatist provinces that is still controlled by Kiev, and rebel leader Alexander Zakharchenko announced shortly after the rocket strikes that the insurgents were launching an offensive aimed at taking it.
On Friday, he had ruled out peace talks and vowed to advance to the western border of the Donetsk region, a major industrial hub where rebels already control swathes of territory.
"Now we have a very interesting marker coming up," said Paul Quinn-Judge of the International Crisis Group think tank in Brussels. "Will the Russians rein them in? Obviously, you've got clear signs of either (they will or they won't) right now."
Mariupol, part of the Donetsk region, provides a land bridge between rebel-held regions to the east and the Black Sea peninsula of Crimea, which Moscow annexed from Ukraine in March and where an important Russian naval fleet is based.
It has recently seen only sporadic violence, but was previously hit by heavy combat on its outskirts, with a rebel assault in early September leading Kiev to repel the attack, but at a heavy cost.
The fighting prompted President Petro Poroshenko to pursue a September truce left in tatters by the recent violence.
Despite the rebels' belligerence, it was not yet clear whether they were prepared to mount the kind of large-scale offensive necessary to take Mariupol.
Analysts speculated that the rocket assault may have been meant to sow panic and degrade confidence in Ukraine's government, or possibly distract Kiev's military while rebels pursue their advance elsewhere.
Oleksiy Melnyk of the Razumkov Centre think tank in Kiev said he believed the aim was "to break the morale of the Ukrainian authorities, so as to provoke social unrest or protests in frontline regions."
He alleged that, for Moscow and the rebels, "the purpose of the firing was to show Ukrainians that the military did not protect the population".
The potential danger is apparent. While Kiev's retreat from the Donetsk airport was merely a symbolic loss since the site was in ruins after months of fighting, Mariupol's surrender would be devastating.
Oleksandr Sushko of the Institute for Euro-Atlantic Cooperation in Kiev pointed out that the city is the largest Ukrainian port on the Sea of Azov and home to two major steel plants.
"The value of Mariupol is much more than symbolic," he said of the city of some 500,000 people.
While its intentions are far from clear, Russia may indeed see the current situation as an opportunity to consolidate territory, US-based security analysts Stratfor said.
"The Russians do not want a massive confrontation with the West at a time of economic dysfunction, yet at the same time, having made the decision to hold Crimea, they will not have a better moment for consolidation," it said.
Russia, under heavy Western sanctions over its alleged role in Ukraine, has repeatedly denied supplying troops, weapons and equipment to the rebels despite satellite images to the contrary.
Support for the sanctions had been seen as weakening among some European nations that rely on Russia for trade, but with peace efforts tossed aside by the rebel offensive, both the United States and EU have again threatened consequences for Moscow.
Russian President Vladimir Putin has so far shown little will to compromise, even with sanctions and low oil prices taking a heavy toll on his economy.
Some analysts have described Moscow's strategy as seeking to create a "frozen conflict" allowing Russia to maintain influence, while at the same time draining Kiev's defence and economic resources, preventing further integration with the West.
Ukraine is already struggling through a severe financial crisis and requires an estimated $15 billion from world lenders this year.
"If they have a strategy, that's a plausible guess," Quinn-Judge of the International Crisis Group said of Russia.
"A lot of the time, they seem to be winging it, which is the disturbing thing about it."
By Mike Smith (AFP)