According to a White House statement last night, the Prime Minister updated Obama on his negotiations with EU leaders during a phone call yesterday. The White House said Obama "reaffirmed continued US support for a strong United Kingdom in a strong European Union".
Obama has previously indicated he wants the UK to remain in the EU, telling the BBC last summer that the UK's EU membership "gives us much greater confidence about the strength of the transatlantic union".
The US President is expected to visit the UK later this spring, and remain campaigners are hopeful that he will repeat the pro-EU message during the trip.
Obama and Cameron also discussed Syria and tomorrow's "Supporting Syria" conference in London. Obama will not attend the conference, which is being co-hosted by Cameron to drum up aid support for refugees.
US secretary of state John Kerry is expected to represent the US government at the event.