The threats came in statements issued after a video conference that brought together Obama and the leaders of Britain, France, Germany and Italy, as well as the head of the European Council.
The virtual meeting took place amid continuing violence in Ukraine despite the two-week-old ceasefire accord reached in Minsk.
White House spokesman Josh Earnest said Obama used the call to condemn Russia for not following through on its agreements.
"It was an opportunity ... for the president to condemn the continuing failure of Russia and the separatists it backs to abide by the commitments to which they agreed," Earnest said.
The United States has also said it is still considering the possibility of sending Kiev weapons.
"Leaders will be ready to decide on further sanctions if the Minsk agreements are further violated," said a European Union statement that was echoed by one from German Chancellor Angela Merkel's spokesman.
The office of French President Francois Hollande said the leaders agreed that a major breach of the agreement would make a "strong reaction from the international community" necessary.
The video conference overlapped a speech before a joint session of Congress by Israeli Prime Minister Benjamin Netanyahu.
The speech stirred controversy because it was orchestrated by Obama's Republican rivals.
The French statement also said the parties wanted to bolster the resources of monitors from the Organization for Security and Co-operation in Europe (OSCE), a European rights and security watchdog.
European leaders on Monday said they agreed that the OSCE needed a broader role as observers of the ceasefire agreement and of the removal of weapons.
Ukraine and Western governments have accused Russia of sending troops and weapons to support separatists in eastern Ukraine despite the peace deal reached on Feb. 12.
Moscow, as usual, has denied providing such support.
On Tuesday, Kiev announced its highest casualty toll in several days, with three Ukrainian servicemen dead and nine wounded amid pro-Russian shelling.
Later this week, Ukraine's parliament is expected to back Ukrainian President Petro Poroshenko's request for international peacekeepers to monitor the conflict.
That idea, however, has faced a chilly reception in Europe.
Source: Google News