DONETSK, Ukraine - Thousands of panicked refugees are flooding highways and packed trains heading out of the main remaining rebel strongholds in eastern Ukraine fearing attacks by government forces who lost 30 soldiers to defiant militants.
A woman carries plastic bags with food as she walks in front of Ukrainian tanks in the small eastern city of Seversk on July 12, 2014
Pro-Russian separatists killed 19 troops in a hail of heavy rocket fire on Friday near the Russian border in a bloody reminder of their resolve to reverse the recent tide of government gains across the east of the country.
Four other troops died elsewhere on Friday and seven more were killed overnight in attacks that also left more than 120 soldiers wounded, the military said.
Ukraine's new Western-backed leader vowed to step up the push east and hunt down the militias responsible, which could shatter all hopes of a truce.
"The rebels will pay for the life of every one of our servicemen with tens and hundreds of their own," President Petro Poroshenko told an emergency security meeting.
US Vice President Joe Biden called Poroshenko on Saturday and offered condolences for the soldiers' deaths, the White House said.
Biden also told the Ukrainian president of "ongoing US diplomatic efforts to work with our international partners to impose costs on Russia if it continues on its current course of providing the separatists with heavy weapons and equipment," it said.
Moscow and Kiev Saturday traded accusations of sparking a shooting incident near the border, with Russia saying it "reserves the right" to defend its territory, while a Ukrainian spokesman said Russian troops were three kilometres (two miles) from the border and might be trying to set up a supply corridor.
- 'Spontaneous evacuation' -
Poroshenko's militant talk convinced many in the million-strong eastern industrial hub of Donetsk -- where gunmen who have been abandoning surrounding cities since last weekend have been retreating -- that their city was about to be bombed.
As many as 70,000 people have already left the city, according to the self-proclaimed rebel prime minister Alexander Borodai.
The mayor of Donetsk rushed out to meet Poroshenko on Friday to discuss measures that could "avoid bloodshed and the use of air strikes and heavy artillery".
But separatists in control of Ukraine's coal mining capital said locals were not taking any chances after three months of fighting that has claimed 550 lives and sparked the worst East-West standoff since the height of the Cold War.
Rebel commander Igor Strelkov told reporters that a "spontaneous evacuation" was also under way in the neighbouring separatist bastion of Lugansk.
AFP journalists on the ground reported sporadic artillery fire around Lugansk, whose streets were deserted, with the empty bus station hit in a mortar attack on Saturday.
"I would say that one car in five is filled with refugees," said a young separatist volunteer manning a roadblock around 20 kilometres (12 miles) east of Donetsk.
Poroshenko had last Saturday proclaimed the seizure of Slavyansk -- the symbolic heart of the uprising -- a turning point in a conflict set off by the ousting in February of Ukraine's former Kremlin-backed president, Viktor Yanukovych, and Russia's subsequent seizure of Crimea.
In a rare move, EU leaders this weekend joined Russia in trying to dampen Kiev's newfound bravado and convince Poroshenko to launch direct truce talks with the separatists.
The EU said Saturday that it was also adding 11 separatist leaders to the names of 61 Russians and pro-Kremlin Ukrainians blacklisted for their roles in enflaming the conflict.
- No Rio meet -
Poroshenko's top aide meanwhile has said that all talks with the rebels were off.
"Those who call themselves leaders of the People's Republics of Donetsk and Lugansk are nobodies -- they are puppets, servants of the Kremlin," presidential administration chief Yuriy Lutsenko told Kiev's Inter television.
"The only possible side that can be involved in negotiations is Russian President Vladimir Putin," he said.
The possibility of such talks being held as early as Sunday had emerged when the Brazilian government said the Ukrainian leader had accepted an invitation to attend the World Cup football final in Rio de Janeiro, where Putin will also be present.
But overnight Poroshenko's office said he would not being going to Brazil and possibly encountering the Russian strongman due to the crisis at home.
"Given the current situation in Ukraine, the head of state decided that it is impossible for him to attend the World Cup final," it said.
For Poroshenko, Ukraine's security headaches have been compounded by the threat of the country going bankrupt if it fails to quickly adopt deeply unpopular austerity measures demanded under the terms of an emergency Western bailout deal.
An IMF team was due to leave Kiev on Saturday after checking whether Ukraine had done enough to merit the second tranche of a $17 billion (12.5 billion euro) loan as part of a broader $27 billion international package.
Standard and Poor's delivered a rare dose of good news to Poroshenko by revising to "stable" from "negative" Ukraine's credit rating based on the conviction that the IMF would not abandon Kiev at this stage.
"Full disbursement of the International Monetary Fund (IMF) programme and related multilateral lending should enable Ukraine to meet its external financing needs over the next year," S&P said.
The two-year international programme is meant to make up for a $15 billion package Russia had extended to former president Yanukovych for his November decision to ditch a historic EU trade and association pact.
His overthrow and the new government's signature of the European deal helped provoke the ongoing insurgency that saw Russia withdraw its aid.