How to create index of unique elements of a column in R dataframe

Question

I have this table here, ordered by country:

country	city
Canada	Montreal
Canada	Toronto
France	Paris
France	Lyon
Australia	Melbourne
Japan	Nagoya

I want to create an index col of countries, something like:

index	country	city
1	Canada	Montreal
1	Canada	Toronto
2	France	Paris
2	France	Lyon
3	Australia	Melbourne
4	Japan	Nagoya

I know I can create another df with distinct countries and then merge. I'm looking for something that can be includide in a pipeline.

Here is a reproductible code:

tibble(country = c('Canada','Canada','France','France','Australia','Japan'),
city = c('Montreal','Toronto','Paris','Lyon','Melbourne','Nagoya'))

Thanks!