List rearrangment with joining of strings

I have a list of strings:

lis = {{"a","b","x"},{"d","e","y"},{"a","b","z"},{"d","c","x"},{"a","b","w"}}

I would like to use StringJoin on the last elements where initial elements are the same to obtain:

res = {{"a","b","xzw"},{"d","e","y"},{"d","c","x"}}

Doing:

lis = SortBy[lis, {#[[1]]} &]SequenceCases[lis, {{a_, b_, c_}, {a_, b_, d_}} :> {a, b, StringJoin[c, d]}]

only gives:

{{"a", "b", "wx"}}

Thanks for any suggestions.