Ukraine has accused Russia of shelling the country from across the border as US Vice President Joe Biden visited Kiev on the first anniversary of mass protests which triggered the past year of turmoil.
During his visit on Friday, Biden warned that Moscow was risking isolation over its "unacceptable" aggression in Ukraine.
"It's simply unacceptable in the 21st century for countries to attempt to redraw borders by force ... because they don't like a decision their neighbour has made," he said after meeting Ukrainian President Petro Poroshenko.
Biden, who left for Turkey a few hours after his arrival in Kiev, warned that the Kremlin had failed to fulfil its commitments under agreements struck in September, adding: "So long as that continues, Russia will face rising costs, greater isolation."
Ukraine's government hoped Biden would use his visit to announce further US assistance for its forces, locked in a drawn-out struggle with pro-Moscow separatists in eastern Ukraine.
The US has so far limited its support to non-lethal security assistance but Kiev wants it to go further and offer weapons and ammunition.
As Biden met Poroshenko and Prime Minister Arseniy Yatsenyuk on Friday, Kiev claimed that shelling was taking place from across the Russian border for the first time since a tattered ceasefire was signed in September.
Ukrainian military spokesman Andriy Lysenko said that in the past day, artillery was fired at a border post in Luhansk region from the direction of Manotsky, in Russia's Rostov region.
In Kiev, dozens of people gathered at the iconic Independence Square, known locally as Maidan, laying flowers at shrines to the more than 100 people who died in protests that started on November 21 last year.
The demonstrations started last year after then President Viktor Yanukovych scrapped a deal for closer ties with Europe.
The protests eventually led to his fall in February which prompted Moscow to seize Ukraine's Crimea peninsula and later triggered separatist unrest in the industrial east which has killed more than 4,300 people since mid-April.
Al Jazeera's Harry Fawcett, reporting from Donetsk, said there was no sign of the day being marked in the separatist region.
"People here are gathering for humanitarian aid," he said. "They are non-plussed by the idea of this being commemorated in this way.
"The leadership here is saying the visit of Joe Biden undermines the Minsk truce and risks re-escalating the conflict."
Poroshenko was heckled by relatives of the Maidan dead shouting "Shame!" over authorities' failure to convict anyone in connection with the deaths when he laid a candle at the shrines on Friday.
Yatsenyuk said on Thursday he hoped for an announcement on further US assistance to Ukraine during Biden's visit following a $53m package announced in September, which included $46m of security assistance.
That included non-lethal military equipment such as night vision goggles, body armour and radios, but Kiev wants Washington to provide lethal assistance.
Pentagon spokesman Rear Admiral John Kirby told reporters on Thursday that the US was "still focused on non-lethal assistance right now".
Russia has warned against the US arming Ukrainian forces, with the secretary of Russia's national security council, Nikolai Patrushev, saying the conflict in eastern Ukraine "will grow" if this happened. Moscow denies arming the separatists.
Source: Al Jazeera and agencies