Can I travel to Belarus or Ukraine with a valid Russian visa?

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.

Since 2020-07-01, you can visit Belarus with a Russian visa and vice versa. See Russia, Belarus sign agreement on mutual recognition of each other's visas.

You cannot visit Ukraine with a Russian visa, unless you also have a Ukrainian visa.

See also this post about border crossings between Russia and Belarus.

Warning as of 2020-07-07: another source in Russian, dated 2020-07-01, states it will take several more months until it actually takes force. These sources contradict each other. Either it's already in force, or it will be soon (and as of July 2020, the 2019-2021 Coronavirus Pandemic means few travellers should make the overland EU-Belarus-Russia trip anyway, with the Russia-Belarus border apparently closed for everybody).