Since the beginning of his presidency, President Biden only addressed the Iranian nuclear issue once, in his first year, eager to reestablish Obama's deal in 2015. Since then, nothing concrete has been done, there is no policy, as the Foreign Affair mag states in an article from 03/26/24.
Since the US, which has been the most vocal country at the P5+1 against Iran, is in silence and apart from the deal, the chances of snapback sanctions drop considerably.
Russia is too occupied with the Ukrainian war. China is "friendly", recently accepting Iran at the BRICS. Germany's Chancellor is too conservative on Public Affairs. The UK tends to follow the American foreign policy regarding Middle-Eastern issues. The most "inclined" to do such a move would be France, whose President recently scaled up the tone against Russia and has been trying to affirm the country as a larger player at the international arena. However, this is still a very remote chance.
However, if Iran can enrich Uranium to over 90%, the scenario is different and it's way more likely that the UK, France, and Germany propose snapback sanctions and further retaliation. Unfortunately, this scenario is closer than previously imagined, since the Institute for Science and International Security (ISIS) has just concluded that Iran has reached "breakout time", meaning it would take just a few days to build a nuclear bomb with the technology they currently have.
According to the ACLED, since 24 February 2022, Russia has caused Ukrainians in Kyiv 129 kills and 511 injured people, meaning 640 victims in total. This represents approximately 24 victims in Kyiv per month.
The possibility of a larger attack from Russia that increases that number to 100 in a single strike, in the next 30 following days (before June 1st ), is not absurd.
Ukraine has had an increasing vulnerability since the Middle East conflict became a priority to the West (USA and UE), after the Hamas attack against Israel (7 October), and even more after the drone and missile strikes from Iran against Israel (14 April).
If that situation escalates, it might represent an opportunity for Russia to impose even more pressure on Ukraine through larger strikes over Kyiv and try to impose a partial victory faster.
However, the time span is considerably short (30 days), therefore the chances are lower. In a larger period (e.g., 12 months) an attack with over 100 victims in Kyiv may even be the most likely scenario.