Ukraine's government has vowed to continue its military offensive against separatists in the country's eastern regions, following days of deadly clashes.
President-elect Petro Poroshenko, who scored an overwhelming first-round victory in a poll on May 25, swore on Friday to punish those responsible for the shooting down on Thursday of an army helicopter near Slovyansk, which killed a high-ranking general and 13 soldiers.
Acting Defence Minister Mykhilo Koval, repeating charges that Russia was carrying out "special operations" in the east of Ukraine, said that Ukrainian forces would continue with military operations in border areas "until these regions begin to live normally, until there is peace".
He said that government troops have "cleaned up the separatists" in the southern and western parts of the Donetsk region, and the north of the Luhansk region.
In the provincial government headquarters in Donetsk, separatists have set up defences outside of the building, including an anti-aircraft gun, indicating that they are preparing for an assault by Ukrainian troops, Al Jazeera's John Wendle, reporting from Donetsk, said.
In Slovyansk, another pro-Russian rebel stronghold, residents continued to flee as fighting intensified between separatists and Ukrainian security forces.
Random shelling and stray bullets have already taken the lives of dozens of civilians although it is difficult to get any precise numbers.
Early on Friday, a local hospital was damaged in a mortar attack. Some of the debris that was sent flying when the mortar round hit also damaged a nearby building used by medical staff to diagnose children's diseases.
The self-declared mayor of Slovyansk, Vyacheslav Ponomarev, a pro-Russia militiaman, acknowledged that the area was unsafe and said that residents should make plans to leave.
Ukrainian authorities have long alleged that the revolt in the east have been fomented by Moscow.
Reports by Ukrainian border authorities and journalists on the ground appeared to show increasing evidence of direct involvement by volunteer fighters from Russia in the rebellions that erupted two months ago in the wake of Russia's annexation of Crimea.
According to these reports, fighters may be coming into Ukraine from former hotspots in Russia and its North Caucasus fringes such as Chechnya whose own troubles in the past 20 years have spawned a proliferation of armed groups.
Ukraine's authorities said Russian border guards are doing nothing to stop fighters crossing the long land border from Russia, along with truck loads of ammunition and weapons.
In the latest such report, Ukrainian border guards said on Friday they had seized a cache of weapons including guns, machine-guns, grenade-launchers, sniper rifles and 84 boxes of live ammunition in two cars they stopped as they crossed from Russia.
Al Jazeera's David Chater, reporting from Donetsk, also said that separatist fighters were seen hauling weapons and heading towards the Donetsk regional airport.
Meanwhile, the Organisation for Security and Cooperation in Europe (OSCE) has reported that it lost contact with a five-member team in Luhansk, where four members of another OSCE mission are still being held by pro-Russian rebels.
The OSCE said it lost contact with the team, made up of four international workers and one Ukrainian translator.
An insurgent leader in the Donetsk region confirmed on Thursday that members of the first group were in their custody.
The OSCE's teams are in Ukraine to monitor the security situation following Russia's annexation of Crimea and the rise of the pro-Russia separatist insurgency in the east.