WASHINGTON — The Obama administration is struggling to cut off the millions of dollars in oil revenue that has made the Islamic State in Iraq and Syria one of the wealthiest terror groups in history, but so far has been unable to persuade Turkey, the NATO ally where much of the oil is traded on the black market, to crack down on an extensive sales network.

Western intelligence officials say they can track the ISIS oil shipments as they move across Iraq and into Turkey’s southern border regions. Despite extensive discussions inside the Pentagon, American forces have so far not attacked the tanker trucks, though a senior administration official said Friday “that remains an option.”

In public, the administration has been unwilling to criticize Turkey, which insists it has little control over the flow of foreign fighters into Iraq and Syria across its borders, or the flow of oil back out. One senior official, calling President Obama’s recent conversations with Turkey’s president, Recep Tayyip Erdogan, “sensitive,” said the decisions about what the country will do to counter ISIS “will be theirs to make.”

But behind the scenes, the conversations about the Sunni extremist group’s ability to gather vast sums to finance its operations have become increasingly tense since Mr. Obama’s vow on Wednesday night to degrade and ultimately destroy the group.