(Adds Biden call to Poroshenko, paragraphs 10-11)
WASHINGTON, June 23 (Reuters) - President Barack Obama urged Russian President Vladimir Putin on Monday to support peace in Ukraine and said Moscow would face further sanctions if it did not take steps to reduce tensions in the country, the White House said on Monday.
"The president spoke to President Putin and once again urged him to support peace instead of allowing the provision of arms and materiel across the border and continuing support for militants and separatists who are further destabilizing the situation in Ukraine," White House spokesman Josh Earnest told reporters.
"Russia will face additional costs if we do not see concrete actions to de-escalate the situation," he said of Obama's message to Putin.
The two leaders spoke in a call earlier on Monday.
Putin and Obama discussed proposals for peace in Ukraine, after Ukraine's leader announced a ceasefire in military operations in the east, the Kremlin said in a statement.
"They discussed ... the implementation of the peace plan proposed by (Ukrainian) President Petro Poroshenko," the Kremlin said. "Putin stressed that priority must be given to halting military operations and to the start of direct negotiations between the opposing sides."
The White House said in a follow-up statement that Obama welcomed Poroshenko's peace plan and pressed Putin for Russia to work toward implementing it.
"The president called upon President Putin to press the separatists to recognize and abide by the ceasefire and to halt the flow of weapons and materiel across its border into Ukraine," the statement said.
"The president emphasized that words must be accompanied by actions and that the United States remains prepared to impose additional sanctions should circumstances warrant, in coordination with our allies and partners."
U.S. Vice President Joe Biden told Poroshenko in a phone call on Monday that the United States would continue to strongly support his peace plan, including the disarmament of all illegally armed groups, release of hostages, and de-occupation of government buildings, the White House said in a statement.
Poroshenko underscored to Biden the importance of Ukraine having full control over its border to prevent the inflow of armed militants and weapons, which continue to cross into Ukraine from Russia, the White House said.
Obama and Putin also discussed the removal of chemical weapons from Syria and efforts to ensure Iran's nuclear program was peaceful. (Reporting by Jeff Mason; Editing by Leslie Adler and Eric Beech)