David Cameron and Barack Obama have discussed concerns about the lack of "concrete action" from Russia over the ongoing crisis in Ukraine.
In a telephone call the Prime Minister and US President underlined their commitment to implement further sanctions on Moscow unless it takes firm measures.
The discussion took place shortly after Russian President Vladimir Putin asked the Moscow Parliament to cancel a resolution sanctioning the use of military force in the country - a move which was welcomed by Ukrainian President Petro Poroshenko as a "practical step" towards peace.
Later this week the signing is expected of remaining elements of an association agreement between the European Union and Ukraine. It was former president Viktor Yanukovych's refusal to sign the agreement which sparked the protests leading to his overthrow earlier this year.
Also to be signed at the Brussels summit of the European Council are similar association agreements with former Soviet states Moldova and Georgia.
A No10 spokeswoman said: " President Obama called the Prime Minister to discuss the latest developments in Ukraine and Russia's response. Both leaders welcomed the efforts taken by President Poroshenko to establish a ceasefire in Eastern Ukraine.
"They noted that while President Putin had called on the separatists to accept the ceasefire, we had yet to see concrete action from Russia to halt the flow of weapons across the Ukrainian border and to stop Russian training of separatist groups.
"This is what the G7 had called on Russia to do and they reaffirmed their commitment to implementing further sanctions on Russia if these things don't happen. The Prime Minister noted that he would discuss this issue with other European leaders at the European Council and would press for preparatory work on such sanctions to continue."