Three days ago, the United States imposed a new round of sanctions that could punish any foreign country that buys Iranian oil. However, it has issued six-month exemptions to 20 importers of Iranian oil who have significantly cut their purchases, including China, which has openly opposed the pressure on Iran. …

Still, President Obama and his European allies — with little help from the Chinese, who actually increased their purchases of Iranian crude in May — are placing a bet that another big turn of the economic screws may change Iran’s attitude.

“It is our assessment that the Iranians have not experienced deep enough sanctions, long enough to fully understand what their isolation means,” a senior administration official closely involved in strategy said Friday in an interview.