Ukrainian President Petro Poroshenko says the threat of war is "hanging over" the country as cease-fire violations continue in eastern Ukraine.
Poroshenko, speaking at an international conference on Ukraine in Kyiv on April 28, said the country needs aid and "solidarity" to prevent war from "erupting" and to resolve the situation in the Donbas region where Russian-backed separatists control territory.
More than 6,100 people have been killed in the past year as government forces have battled the rebels.
Poroshenko's comments came as military officials said the separatists had resumed using rocket launchers that are banned under a February cease-fire agreement signed in Minsk.
The military said in an April 28 statement that rebels used Grad rockets the previous evening against Ukrainian Army positions in Avdiyivka, near the separatist-held provincial capital of Donetsk.
Military officials said that one soldier had been killed and 14 others wounded in eastern Ukraine in the previous 24 hours.
Eduard Basurin, a self-styled representative of separatist military forces in rebel-held parts of Ukraine's Donetsk region, in turn accused Ukrainian forces of numerous cease-fire violations.
In southeastern Ukraine, the Organization for Security and Cooperation in Europe (OSCE) said in a statement on April 28 that rebel fighters have refused three successive times to allow OSCE monitors access to separatist-held areas near the town of Shyrokyne.
Responsibility for checking whether the cease-fire deal is being implemented lies with the OSCE.
The OSCE said on April 26 it had recorded the fiercest fighting at Shyrokyne since the cease-fire was signed two months ago.
The small town is less than 20 kilometers from the strategically important Ukrainian seaport of Mariupol, which some rebel leaders have vowed to capture.
The OSCE said it saw dozens of tank shots and plenty of other weapons proscribed under the peace deal during clashes in Shyrokyne on April 26.
In Washington, U.S. State Department spokesman Jeff Rathke said on April 28 that Moscow has recently deployed more air-defense systems in eastern Ukraine, including some near front lines.
Meanwhile, U.S. Vice President Joe Biden said in a video address at the Kyiv conference that economic sanctions against Russia should remain in effect until the complete fulfillment of the Minsk agreement, including the restoration of Ukrainian control over all of its border crossings with Russia that are currently in rebel hands.
He said the sanctions were necessary to keep pressure on Moscow, which he said had "occupied" Ukrainian territory and sent troops, mercenaries, tanks, and missiles to eastern Ukraine.
Biden said "the Russian aggression" had caused a huge humanitarian disaster in which some 2 million Ukrainians had left their homes.
Russia has said it has no troops in Ukraine despite claims by several western countries, NATO, and Kyiv that there is clear evidence of their presence.
Ukrainian Prime Minister Arseniy Yatsenyuk on April 28 urged delegates at Kyiv's international conference to step up its financial commitment to Ukraine.
He noted that Ukraine had received only 10 percent as much money from the IMF and Group of Seven (G7) as Greece, which he said received some $300 billion in loans, even though Athens has no war or "Russian tanks" on its territory.
With reporting by Interfax and AP