DOHA, Aug. 26 -- Qatar's foreign ministry delivered a carefully worded rebuke of Washington on Tuesday, calling the United States' sweeping new economic campaign against Iran "unilateral," as the Gulf state's peculiar role as mediator between the two countries showed signs of strain under the weight of a sanctions package designed not to negotiate but to end.

Foreign Ministry spokesman Majed al Ansari used his weekly press briefing to register Doha's discomfort without crossing into direct criticism of its most powerful military tenant. Al Udeid Air Base, the largest American installation in the Middle East, sits on Qatari soil. The contradictions that creates have defined Qatari foreign policy for years, but they have rarely been sharpe...