Russia signaled Friday that a deal could be clinched in Iran nuclear negotiations, as there is "political will" and a sense of urgency among participants for an accord before a July 20 deadline.

"One feels political will of the participants, and a certain fear that we may not be quick enough -- that is a good sign in this situation. There is not much time left. But there are chances," Russia's deputy foreign minister Sergei Ryabkov told Kommersant newspaper.

Gunmen dressed in camouflage shot dead three Ukrainian policemen and wounded another in the restive eastern city of Donetsk on Thursday, the regional interior ministry said.

The bodies of the officers were found near the railway in the northwest of the industrial city, which is also the administrative center of the separatist region of Donetsk.

Russian President Vladimir Putin on Thursday expressed "deep concern" over the growing number of civilian casualties and refugees in eastern Ukraine.

"Vladimir Putin expressed deep concern over the growing number of peaceful civilians and a sharp increase in the number of refugees from southeastern Ukraine in Russia," the Kremlin said in a statement after a telephone conversation between Putin, German Chancellor Angela Merkel and French President Francois Hollande.

Fighting rumbled on in east Ukraine Thursday between government troops and pro-Russian rebels despite a fresh push for ceasefire talks from international envoys. 

Ukraine's border service said that nine guards were wounded when rebels shelled the Dolzhansky border post with Russia, a strategic crossing that Kiev won back days earlier in a step hailed as the "first victory" since the renewal of its military offensive.

A former policeman who headed the state's VIP bodyguard service in Ukraine was Thursday named the country's new defense minister, making him the pointman in the battle against pro-Kremlin rebels in the east.

Valeriy Geletey was nominated to the post by President Petro Poroshenko and confirmed by parliament.

The foreign ministers of Ukraine, Russia, Germany and France agreed Wednesday that fresh ceasefire talks involving the OSCE should start no later than Saturday on crisis-torn Ukraine.

"Ministers urge that the Contact Group should resume no later than July 5 with the goal of reaching an unconditional and mutually agreed sustainable ceasefire," the four top diplomats said in a joint statement after talks in Berlin.

Ukrainian troops backed by tanks and fighter bombers suffered their first losses on Wednesday as they pressed on with a renewed offensive against pro-Kremlin insurgents that has drawn Russian ire but also vital U.S. and German support.

The return of all-out fighting in Europe's worst security crisis in nearly two decades set off a new international scramble to dampen hostilities in the strategic ex-Soviet state.

German Chancellor Angela Merkel pledged Wednesday no let-up in efforts to find a diplomatic solution to the Ukraine crisis, ahead of foreign ministers' talks later in Berlin.

Merkel told a joint press conference with NATO head Anders Fogh Rasmussen that it was "very regrettable" that the 10-day ceasefire proclaimed by the Ukrainian government had not ended the violence.

U.S. Secretary of State John Kerry on Tuesday pushed Russia to stop backing pro-Moscow insurgents in Ukraine, as the West mulled further sanctions against the government of Vladimir Putin.

In a call to Russian Foreign Minister Sergei Lavrov, Kerry "expressed strong concern about the refusal of Russian-supported separatists to take the necessary steps" to enable the extension of a shaky 10-day ceasefire.

The Ukrainian and Russian foreign ministers will hold talks Wednesday in Berlin with their French and German counterparts in a new push to tackle the crisis in the ex-Soviet state, a French diplomat said.

Pavlo Klimkin, Sergei Lavrov, Laurent Fabius and Frank-Walter Steinmeier will meet Wednesday afternoon, the diplomat said as Ukrainian forces resumed their assault on pro-Russian insurgents following the expiry of a 10-day truce.