There are mounting signs that diplomacy with Iran over its nuclear program is running into trouble. The point of the interim accord negotiated last year, one of the Obama administration’s most important diplomatic achievements during its second term, was to buy time so that a more comprehensive agreement to rein in Iran’s nuclear program might be reached.

But the recent round of formal negotiations between six world powers and Iran in Vienna, aimed at producing a comprehensive agreement, made scant progress. Each side accused the other of being intransigent. With no progress to announce, Catherine Ashton, the senior foreign policy official for the European Union, chose to dispense with the customary joint news conference with her Iranian counterpart, Mohammad Javad Zarif.

The interim accord, which is due to expire on July 20, has a clause that allows for a six-month extension. But many American lawmakers are skeptical of the Iranians’ intentions. Some negotiating progress would likely be needed to extend the talks for several more months and forestall congressional pressure for additional sanctions.