Vice President Joe Biden offered condolences to the Ukrainian president Tuesday after rebels in the eastern part of the country downed a military helicopter, killing nine servicemen.
The violent attack came one day after pro-Russian insurgents in the conflict-torn region said they would agree to a ceasefire declared by President Petro Poroshenko.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
Sign up for breaking news alerts from NBC News
The cease-fire is set to expire Friday, but Poroshenko warned Tuesday that he may terminate it early over the downed helicopter.
In his conversation with Poroshenko, Biden underscored the importance of having monitors in place to verify violations of the cease-fire, according to a statement from the White House.
Echoing that stance, President Obama and British Prime Minister David Cameron re-emphasized on Tuesday the need for Russia to stop the flow of weapons and militants across its border with Ukraine. The two leaders also believe Russia should exercise its influence over the insurgents to persuade them to embrace Poroshenko's peace plan.
Should Russia not take steps to deescalate the violence in eastern Ukraine, "the United States and the European Union would work to implement additional coordinated measures to impose costs on Russia," the statement read.
Reuters contributed to this report.