Negotiators have made significant progress in the last week or so on an agreement to revive the 2015 Iran nuclear deal but very tough issues remain, a senior US State Department official has said.

“There’s been significant progress over the last week or two. We have significantly narrowed the areas that still need to be resolved. And so, in that sense, there has been progress that has been moving toward a potential deal,” he told reporters.

“So we are in a better position than we have been. But at the same time it’s important to note that very serious issues remain.”

The aim of the negotiations is to return to the original 2015 bargain of lifting sanctions against Iran, including ones that have slashed its oil sales, in exchange for restrictions on its nuclear activities that extend the time Tehran would need to make enough enriched uranium for an atomic bomb if it chose to.

Iran’s lead negotiator, Ali Bagheri Kani, has gone back to Tehran and the senior US official said he hoped the Iranian official would return to Vienna, where the talks are held, in a positive frame of mind. However, he said that there would still be some issues to settle even after Bagheri Kani’s return.

The US official also said that there has not been any deal reached in separate negotiations about the release of four US citizens whom the US believes have been wrongfully detained by Iran.

Meanwhile, an Iranian activist went missing after criticizing a proposed bill by hard-liners to implement highly restrictive internet policies, his family said on Saturday.

Hossein Ronaghi, a blogger and free-speech activist, disappeared Wednesday after he criticized a bill in parliament to limit internet access in the country, known as the “Users Protection Bill.” The proposal has been criticized by many Iranians on social media.

There was no information on Ronaghi’s location or condition.

Iran’s supreme leader Ayatollah Ali Khamenei, who has final say on all state matters, said in March last year that social media in Iran is “unbridled” and it should not be “surrendered to the enemy.”

In a recent tweet, Ronaghi said: “The Protection Plan was a decision made by the entire system based on the demand from the Islamic Republic’s leader who had stated: ‘Virtual space must be controlled.’“

Ronaghi’s brother, Hassan, who also is an activist, said in a tweet that Hossein was kidnapped. He said his brother had received several anonymous phone calls in the days leading up to his disappearance.

Hassan Ronaghi also said his brother needs medical care because he is suffering from diseases affecting several of his organs, including his kidneys.

“Anything that happens to Hossein is the responsibility of the Supreme Leaders’ office, the (Revolutionary Guard), and the judiciary.”

Reza Ronaghi, the father of the two brothers, said in an interview with Iranian foreign-base media on Wednesday that Khamenei was directly responsible for his son’s life.

A day after the first reports surfaced of his disappearance, human rights activists claimed that security forces came into Hossein Ronaghi’s home and took a laptop and notebooks.

The language in the proposed internet legislation has yet to be finalized. But if implemented in its current form, it could lead to the disruption of international internet services and websites — like Instagram — that have not yet been blocked.

Under pressure from hard-liners, the Iranian government has long blocked access to many websites and social media platforms, from YouTube and Facebook to Twitter and Telegram.

Many Iranians, especially youths, access social media through VPNs and proxies. Instagram and WhatsApp remain unblocked.