Iran and the other signatories of the 2015 nuclear deal resumed negotiations in Vienna on Thursday to revive the accord, though the atmosphere was fraught in the aftermath of the apparent Israeli attack on a major uranium enrichment site in Iran.
Senior diplomats involved in the talks have agreed that the working groups meant to bring both Iran and the United States into compliance with the deal had made progress.
But after the meeting Thursday, the head of China's delegation, Wang Qun, called for a faster pace and fewer distractions.
"We do think that all these developments have reinforced our conviction that what is needed most now as a top priority is to do away with any disruptive factors and pick up the pace of negotiation here," said Wang, China's ambassador to the International Atomic Energy Agency.
The senior Russian diplomat, Mikhail Ulyanov, said in a Twitter post that the "general impression is positive." He said this meeting would be followed "by a number of informal meetings in different formats, including at expert level."
The talks have been overshadowed in recent days by Iran's response to an attack Sunday at its Natanz uranium-enrichment facility. Tehran decided to further increase enrichment to 60%, a major step toward the 90% enrichment that is considered suitable for a nuclear bomb and a flagrant breach of the limits of the 2015 accord. Iran also said it would replace damaged centrifuges at the Natanz facility with more advanced models that were banned under the accord.
The Natanz attack was said to have been carried out by Israel, which has regularly criticized the 2015 deal as weak and unlikely to restrain Iran's nuclear ambitions. U.S. officials have said Israel was responsible for the attack and have denied any U.S. involvement.
The meeting in Vienna involved senior diplomats from Iran, Britain, China, France, Germany and Russia under the chairmanship of the European Union. Senior U.S. officials are in a nearby hotel.
The three European nations, joined by the United States, have sharply criticized Iran's moves in recent days, calling them "provocative" and "particularly regrettable" in the face of progress at the Vienna meetings.
"Iran's dangerous recent communication is contrary to the constructive spirit and good faith of these discussions," they said in a statement, adding that Iran's enrichment decision was "a serious development since the production of highly enriched uranium constitutes an important step in the production of a nuclear weapon."
On Wednesday, U.S. Secretary of State Antony Blinken criticized Iran's intentions. "I have to tell you, this step calls into question Iran's seriousness with regard to the nuclear talks," he said, "just as it underscores the imperative of returning to mutual compliance" with the nuclear deal.
Iran maintains that its nuclear program is purely civilian.
The talks are designed to bring the United States back into compliance with the 2015 deal by negotiating what economic sanctions should be lifted. A second working group is focusing on how to bring Iran back into compliance, which Iran has deliberately broken as a "remedial" measure since the economic benefits of the accord have been denied.
Although the talks are said to have been positive so far, Iran's supreme leader, Ayatollah Ali Khamenei, was dismissive of them in comments made Wednesday marking the first day of Ramadan in Iran. He said it was "not worth looking at" initial offers for the lifting of sanctions, saying "the offers they provide are usually arrogant and humiliating."
He also warned that time could be running out.
"The talks shouldn't become talks of attrition," Khamenei said. "They shouldn't be in a way that parties drag on and prolong the talks. This is harmful to the country."
He also said Iran was prepared to return to compliance quickly if agreement could be found and again denied that Iran would ever build nuclear weapons.