Thirty foreign ministers -- including from Russia and Ukraine -- were set to discuss the escalating violence at a Council of Europe meeting in Vienna, while UN chief Ban Ki-moon offered to help negotiate a solution before the crisis "spins out of control".
At least four Ukrainian servicemen were killed and 30 wounded battling heavily armed insurgents Monday around the flashpoint eastern town of Slavyansk, and an Mi-24 helicopter was shot down by machine-gun fire. The pilots survived, Ukraine's government said.
The interior ministry in Kiev said the pro-Russian gunmen controlling Slavyansk were using civilians as human shields and shooting from houses, some of which were ablaze.
"They are waging a war on us, on our own territory... my mission is to eliminate the terrorists," Interior Minister Arsen Avakov told reporters near Slavyansk.
The head of Ukraine's national guard, Stepan Poltorak, said: "We have bottled them (the rebels) up in the centre" of Slavyansk, but added that "our adversaries are well-trained and well-equipped".
Authorities in the regional capital Donetsk said one civilian in Slavyansk was killed and 15 wounded in the fighting.
- 'Destructive consequences' -
The advance on Slavyansk was part of a wider military operation in the east to root out the separatist insurgents, who are holding more than a dozen towns as Ukraine is rent by a political crisis that has pitted Russia against the West in a Cold War-style standoff.
Russia, which denies any hand in the violence, warned in a foreign ministry report Monday that the unrest was now "fraught with such destructive consequences for Europe's peace, stability and democratic development that it is absolutely necessary to prevent it".
The report accused Ukrainian "ultra-nationalists" -- who Moscow claims control Kiev's government -- of rights violations on a "mass" scale.
Moscow later warned of an evolving "humanitarian disaster" in eastern Ukraine where it said Kiev was carrying out "terror against its own people".
But Ukraine's interim president Oleksandr Turchynov declared it was Russian meddling that had brought war to his country.
"War is in effect being waged against us, and we must be ready to repel this aggression," he said.
There were concerns also for the south of Ukraine, in the port of Odessa, which was seething after deadly clashes and a fire on Friday that killed 42 people.
- Diplomatic push -
As diplomats scrambled to dial down the tensions in the worst East-West crisis since the end of the Cold War, the UN chief offered himself as a personal go-between.
Speaking exclusively to AFP in Abu Dhabi, Ban offered "to provide my own role if necessary" before the crisis "spins out of control and creates huge consequences beyond anybody's control".
Ban's offer came as European leaders, fearing all-out civil war on their eastern flank, launched a new peace bid, urging a negotiated solution.
Russian Foreign Minister Sergei Lavrov and his Ukrainian counterpart Andriy Deshchytsia have arrived in the Austrian capital for Tuesday's Council of Europe meeting which will also include Britain's William Hague.
Hague will travel to Kiev later in the day where parliament will discuss the crisis in a closed-door meeting.
In Washington, US Secretary of State John Kerry is to hold talks with EU foreign policy chief Catherine Ashton.
The chairman of the Organisation for Security and Cooperation in Europe, Didier Burkhalter, is due in Moscow on Wednesday.
German Foreign Minister Frank-Walter Steinmeier said he was in talks with Russia, the United States, the European Union and the OSCE to hold a second peace conference in Geneva.
- Accord dead -
A first agreement aimed at defusing the crisis was signed in the Swiss city on April 17. But Russia last week pronounced the accord dead after Kiev stepped up military operations.
The Kremlin's stance raises the prospect of tougher US sanctions on whole sectors of the recession-hit Russian economy.
French Foreign Minister Laurent Fabius tweeted that Europe was "not responding as it should" to the crisis "because of its energy dependence" on Russia.
Moscow has threatened to turn off the gas taps to Ukraine -- and by extension several European countries -- if Kiev does not pre-pay its June bill.
The separatists in Ukraine are preparing their own spoiler of a presidential election due on May 25 with plans to hold an independence referendum on Sunday.
The presidential vote was called by Ukraine's new leaders shortly after the ouster of pro-Kremlin president Viktor Yanukovych in February, the culmination of months of sometimes deadly pro-EU protests.