The Obama administration initially said that it would pursue a friendly co-operation with Iran provided it reciprocates. And in the last one year, the new US administration has found no change in Iran’s attitude, and more so it has expedited the Nuclear related activities.
So US has now decided to pursue stricter UN sanctions against Iran in the coming days, for which it is in the process of mustering support of the entire world. Russia, China and France are not agreeing to the US measures and are expecting slightly slower pace of sanctions against Iran.
These three countries have commercial relationship with Iran which would be badly hit if the sanctions are passed. Hence US may need to do more to convince these three major countries to agree on new Iran sanctions.