Ukrainian President Petro Poroshenko has warned he may scrap a cease-fire after pro-Russian separatists shot down a Ukrainian helicopter.
Poroshenko's statement on June 24 came after nine Ukrainian servicemen died when an Mi-8 military helicopter was hit by a rocket and crashed near the eastern city of Slovyansk.
That incident comes amid attacks by pro-Russian insurgents on the Donetsk airport and checkpoints that led to several injuries and one death, despite separatist leaders agreeing on June 23 to Poroshenko's unilaterally-declared cease-fire.
Poroshenko declared a one-week cease-fire on June 20 as part of a peace plan to end the fighting which has killed hundreds.
He said insurgents have fired on Ukrainian positions 35 times since then.
Meanwhile, the White House said U.S. Vice President Joe Biden on June 24 called Poroshenko and "offered condolences" for the recent deaths.
Biden also highlighted "the importance of having monitors in place to verify violations of the ceasefire, as well as the need to stop the supply of weapons and militants from across the border."
It was Biden's third call with Poroshenko in as many days.
Poroshenko is also due to discuss the helicopter incident with Russian President Vladimir Putin in a teleconference on June 25 that would also be joined by German Chancellor Angela Merkel and French President Francois Hollande.
Separately, President Barack Obama and British Prime Minister David Cameron discussed possible new sanctions on Russia.
A White House statement said that Obama and Cameron "agreed that should Russia fail to take these immediate steps to de-escalate the situation in eastern Ukraine, the United States and the European Union would work to implement additional coordinated measures to impose costs on Russia."
On June 24, Russian President Vladimir Putin asked the upper house of parliament, the Federation Council, to revoke its authorization for Russian military forces to intervene in Ukraine.
But Putin said that Russia will continue to protect the interests of ethnic Russians in Ukraine.
Lawmakers are expected to withdraw the mandate on June 25.
Poroshenko called Putin's request to lawmakers the "first practical step" by Russia toward settling the crisis.
The move comes one day after EU foreign ministers, meeting in Luxembourg, urged the Kremlin to revoke the March 1 mandate authorizing Putin to deploy Russian troops in Ukraine if he saw fit to do so.
NATO foreign ministers are set to discuss the situation in Ukraine at a meeting in Brussels on June 25.