The US says it will only assist in defense, it won't assist in attacking Iran.  The UK, France and Jordan also assisted in defense.

If Israel attacks Iran, it would be strategic of Russia to take that same position and assist in Iran's defense. Only it is not geared up to do so.  When Bush cancelled the ABM treaty, instead of developing anti-ballistic defenses like the US did, Russia decided to develop hypersonic missiles that evade such defenses. But that has left Russia unable to respond in kind in this current situation -- they can't shoot down Israel's attacks. They are optimized to go on the attack, but that would be a more severe escalation that I don't think they will do until they feel they must.

China will stay out of it for as long as possible. But they are ready in case WW3 really heats up.