Kiev - US President Barack Obama and German Chancellor Angela Merkel will discuss the escalating Ukraine crisis on Friday after Kiev brought back military conscription and a pro-Russian rebellion in the east threatened the ex-Soviet republic with disintegration.
The White House meeting between Obama and Merkel will be their first since the start of the unrest and Russia's annexation of Crimea in March.
Kiev's decision to reinstate conscription, decreed by interim president Oleksandr Turchynov, came after insurgents tightened their grip over more than a dozen eastern cities and towns.
Around 300 militants hurling petrol bombs and bricks stormed the six-storey prosecutor's building in Donetsk, beating up outnumbered riot police and stripping them of their shields and batons.
Ukrainian media reported that a prosecutor's office in the town of Horlivka and a police station in Krasnoarmiysk were also overrun.
The violence took place as mass pro-Russia rallies were held in Donetsk and in annexed Crimea.
Gunfire and heavy detonations were heard Friday just to the north of Ukraine's rebel-held town of Slavyansk, insurgents and AFP journalists said.
A helicopter was earlier also seen circling low in what appeared to be part of an attack by Ukrainian forces, said the rebels, who had set a roadblock on fire to produce cloaking smoke.
Kiev's Western-backed government has admitted its security forces are helpless to halt the expanding rebellion it accuses Moscow of masterminding.
Turchynov on Wednesday accused law enforcement units in the east of "inaction" or even working with the rebels in an act of "treachery".
He also put Ukraine's current army of 130 000 on "full combat alert" because of fears an estimated 40,000 Russian troops massed on the border for the past two months could invade.
In his conscription order on Thursday for Ukrainian male reservists aged 18-25, Turchynov said his government was trying to counter "the deteriorating situation in the east and the south".
The mounting insurgency and building seizures "threaten territorial integrity", a statement from his office said.
Russia's foreign ministry said any effort by Kiev to intensify its military operation "against its own people" in the east could have "catastrophic consequences".
Amid the spiralling crisis, Germany stepped up its appeal to Russian President Vladimir Putin to help free seven OSCE inspectors held in Slavyansk by the rebels - four Germans, a Pole, a Dane and a Czech.
In a phone call Merkel "reminded President Putin of Russia's responsibilities as an OSCE [Organisation for Security and Co-operation in Europe] member and called on him to use his influence", her spokesperson said.
The Kremlin said both leaders emphasised the "mediating potential of the OSCE" in calming the crisis in Ukraine.
German Foreign Minister Frank-Walter Steinmeier was to discuss the hostage crisis with his Swiss counterpart Didier Burkhalter, the current OSCE chief, in Bern on Friday.
Putin reiterated his call for Kiev to end its military operation trying to counter the pro-Russian rebellion.
That drew an incredulous reaction from the White House. A spokesperson said: "That was a rather remarkable statement... [that] called on Ukraine to remove its forces from its country, which is preposterous, if you think about it."
The West, which also believes Putin is pulling the strings in the insurgency despite his denials, this week imposed sanctions on powerful Russian individuals and firms as punishment.
Russia has reacted angrily, but said it would not retaliate unless the pressure was upped further.
Moscow "will not rush to do anything stupid", Russian Foreign Minister Sergei Lavrov said on a trip to Latin America.
The International Monetary Fund has thrown a $17bn lifeline to Ukraine, with $3.2bn of that available immediately.
But the IMF later warned that, "should the central government lose effective control over the east, the programme will need to be re-designed".
The money could be used to pay a $3.5bn Russian gas debt that Putin has warned could lead to him turning off the taps, in a move also affecting several European countries.
Talks were due to take place on Friday in Warsaw between the European Union, Russia and Kiev over the gas dispute.
In Slavyansk, near Donetsk, the week-long stalemate over the fate of the seven detained European military inspectors with the OSCE dragged on. Four OSCE negotiators were seen arriving late on Thursday.
Rebels in the town have been saying for days that the team are about to be freed.
But so far the militants claim only to have exchanged two of three Ukrainian commandos captured separately for some of their colleagues taken prisoner by Kiev, according to the Interfax news agency.
Ukrainian authorities denied that the commandos had been freed.
The condition of all three of the Ukrainian men was unknown. The last time they were seen, on Russian state television, they were savagely beaten, cuffed to chairs with their bloodied eyes bound by tape, and stripped to their underwear.
The unrest in Ukraine, which started with peaceful demonstrations in Kiev against then president Viktor Yanukovych, has rapidly spiralled into one of the worst geopolitical crises in years.
After a deadly crackdown on protesters, Yanukovych was forced out in February, sparking fury in Moscow which led to the Kremlin's blitz annexation of Crimea.
The pro-Russia rebels who have been steadily taking more ground in the east vow to hold their own Crimea-style "referendum" on independence on 11 May - two weeks before a nationwide presidential vote is due to take place on 25 May.