The United Nation is limited to certain things that restrict them from intervening in the recent war. They dont have any right to enforce laws and order.
The Israel and Iran war originates way back in history, its not something the UN can resolved easily. And Russia, as a permanent member of the UN Security Council, has veto power. The veto power restricts strict resolutions against them.

And mind you they are not meant to take any military's action, they are more focused on a diplomatic solution like negotiations to settle dispute.