Can I travel to Belarus or Ukraine with a valid Russian tourist visa?.
I'm a Syrian citizen.
No, and you should not generally expect countries to accept visas issued by other countries.
Ukraine requires Syrian citizens to obtain a visa, which is, as usual, done at a Ukrainian embassy or consulate. Ukrainian visa requirements are here, there is no provision for entering with a visa issued by Russia or any other foreign country.
Belarus likewise requires Syrian citizens to obtain a visa (unless you have a diplomatic Syrian passport), and the Belarusian MFA page on visas (English version) explicitly states that no visas issued by other countries are accepted.