Russia has called on Ukraine's government to stop an operation against pro-Moscow separatists in the east and enter talks aimed at resolving the crisis.
Russia's Foreign Ministry said in a statement that a humanitarian crisis was looming in cities where Ukrainian forces have been trying to dislodge pro-Russian separatists.
It called on the Kyiv authorities "to come to their senses, stop the bloodshed, withdraw forces, and finally sit down at the negotiating table to begin a normal dialogue about ways to resolve the political crisis."
Fighting continued on May 5 in the eastern Ukrainian city of Slovyansk. Large areas of the city have been under the control of pro-Russian separatist forces since last month.
The separatist forces shot down another helicopter of the Ukrainian military, though the Defense Ministry said the pilots got out of the helicopter alive.
The pro-Russian forces shot down two Ukrainian helicopters on May 2, killing two pilots.
Ukrainian Interior Minister Arsen Avakov said four Ukrainian soldiers were killed in the May 5 fighting in Slovyansk and some 30 more were wounded.
Avakov said earlier on May 5 that government troops were facing about 800 militants allegedly using heavy weapons.
Avakov said troops would try to advance "little by little" towards the center of Slovyansk where the pro-Russian rebels have seized the town hall, police headquarters, and the Ukrainian Security Service (SBU) building.
They are also holding several people captive, including the mayor.
The pro-Russian forces told Russian news agencies about 20 of their fighters had been killed and claimed dozens of civilians had been injured.
Kyiv also announced that it was sending police special forces to the southwestern port city of Odesa to replace local police who had failed to tackle pro-Russian activists at the weekend.
Odesa saw some of the worst violence of Ukraine's unrest so far on May 2 when supporters of the government in Kyiv and pro-Russian groups clashed for much of the day.
Pro-Russian supporters fled inside the labor-union building, which caught fire, killing dozens of people inside.
Meanwhile, Russian Foreign Minister Sergei Lavrov and his Ukrainian counterpart, Andriy Deshchytsya, arrived in Vienna on May 5 ahead of a Council of Europe meeting expected to be dominated by the Ukraine crisis.
UN Secretary-General Ban Ki-moon also offered on May 5 to act as a mediator.
Ban said he has been in touch with "all the parties concerned -- Ukrainian leaders, leaders of the Russian Federation, the European Union, and Americans."
Slovyansk, a town of some 140,000, is one of a string of cities in eastern Ukraine controlled by pro-Russian rebels.
The government has vowed to continue an "antiterrorist" operation it launched on April 15 until it regains control. So far, the military offensive has had limited success.
Acting President Oleksandr Turchynov has said Russia "is at war" with his country.
Turchynov, on May 4, also accused Russia of trying to "destabilize the situation completely" in eastern and southern Ukraine.
His comments came after more than 40 people were killed in the southern port city of Odesa on May 2, the bloodiest violence in Ukraine since President Viktor Yanukovych fled the country after he was ousted in February.
Most of the victims died in a blaze after firebombs were thrown into a building where pro-Russia demonstrators had sought refuge amid street fighting.
Pro-Russia activists attending the funeral on May 5 of one of the victims vowed to seize government buildings in Odesa.
Russia denies the Ukrainian government's accusations that it is fomenting the violence in southern and eastern Ukraine.
In a report released on May , the Russian Foreign Ministry warned that failure to halt the escalating unrest in Ukraine would threaten peace across Europe.
It also alleged that there had been rights violations on a "mass scale" by Ukrainian ultranationalists.
With reporting by Reuters, AFP, Interfax, Pravda.com.ua, and dpa