Ukraine claims Russia is using 1.8-ton S300 (SA-10) SAM (surface to air missiles) as SSMs (surface to surface missiles). The allegation was based on Belarus revealing that they had recently tested S300s acting as SSMs. The S300 was known to have been designed with SSM capability. That means one of launch options includes SSM mode. There are two major limitations to using a SAM as an SSM. One is accuracy at longer ranges. Russian SAMs are guided to the target using a ground-based targeting radar that guides the S300 to aerial targets up to 150 kilometers away. SAMs are programmed to self-destruct if they miss their target so that the missile and its small (100-200 kg) warhead does not land on friendly territory. Used as an SSM the self-destruct is disabled and the guidance system aims the missile at a ground location, but less accurately than against aerial targets. Russia has apparently modified the SSM option to include a GPS option that directs the SAM to a specific ground location. The GPS option works, but not as accurately as expected. Ukraine believes that the Russians are running out of SSM missiles and using S300s as SSMs because there is not much need for their SAM capabilities, especially with the more recent and capable S400 now available.
The Ukrainians are also aware of what happened in Israel a year ago when a Syrian S200 missile was deliberately used against a military facility deep inside Israel. This took place in early 2021 when a missile from Syria landed a few kilometers from the Israeli Dimona nuclear research center. There was no damage because Dimona is 300 kilometers south of the Syrian border in a rural desert area. First thoughts were that this was another Iranian attempt to carry out some credible revenge for the recent Mossad operation that destroyed the underground Natanz nuclear fuel enrichment facility. If this was a deliberate missile attack, it failed, but it meant Israel spending a lot of money firing expensive ABM (anti-ballistic-missile) missiles at more Syrian anti-aircraft missiles that cross the border either accidentally or on purpose. Israel is trying to solve this problem by modifying its ABM fire control software to discriminate between the trajectories of SAMs that are not headed for civilian or military targets and those that are. The Israeli Iron Dome system has long used such a method to only shoot down targets headed for targets that must be defended.
Initial examination of the impact area debris near Dimona revealed it was a S200 (SA-5) SAM, a SAM that had been accidently landing for years after missing intended aircraft targets. For example, in 2017 Israel used an Arrow 3 anti-missile missile to intercept a Syrian S200 that had been fired at Israeli jets bombing a target in eastern Syria near Palmyra. Apparently several S200s missed the Israeli jets and instead of detonating anyway, as these missiles are programmed to do, continued into Israeli air space and an Arrow 3 ABM missile was fired just in case the incoming threat was a ballistic missile. At the time it was suspected that Syria might have deliberately modified some of their seven-ton S200 missiles to operate as surface-to-surface missiles. This has been done before with Russian SAMs, usually as an unofficial (and crude) modification by Arab users.
There have been some modern SAMs with a built-in surface-to-surface mode. This was done for the U.S. Nike-Hercules SAMs used during the 1960s and still around. Other users of the Nike-Hercules (like Taiwan and South Korea) made this modification and produced an accurate, if expensive, surface-to-surface short range ballistic missile. The Nike-Hercules was designed for potential use as a surface-to-surface weapon. The U.S. Navy has added a SSM capability to their SM 6 SAM and recently tested it successfully against stationary targets. This mod is meant to give SM-6 an anti-ship missile capability. Earlier U.S. Navy SAMs also had this SSM capability.
The 2017 incident was the first time the Arrow 3 has been used in a combat situation, and it was implied that the Arrow fire control system was programmed to automatically assume that anything resembling a ballistic missile headed into Israel, whether intentional or by accident, was a danger to Israel and should be shot down just to be on the safe side. Syria responded by announcing that if Israel continued carrying out these air raids in Syria they would use hundreds of ballistic missiles (most SCUDs and other Russian designs) against Israel without warning. This was a hollow threat because most Syrian ballistic missiles had been used or lost during the civil war that began in 2012 and is still going on.
Iran has been supporting the Syrians and has the capability to modify S200s for use against targets deep inside Israel. This is what Arrow was designed for and what the Israeli forces constantly prepare for. The recent Dimona incident was promptly claimed by Iran as a revenge attack. Iran also supports the Russian war in Ukraine and can ship cargo to Russia via the landlocked Caspian Sea that borders Russia and Iran. There has been no mention of Iran supplying Russia with mods for their S300s to turn them into more accurate SSMs, but it is a real possibility.