Imputing the missing observations with series mean for panel data in R

I have a panel data that contains some missing values for a number of variables. I want to impute the missing data with series mean for panel data. I tried to use the following code, but I do not know how to ask r to do the calculation by taking into account the year and id or country.

The following code is a trial to impute the missing value for one variable. **My goal is to do this step for all variables.

my_data$V1[is.na(my_data$V1)] <- mean(my_data$V1,na.rm = TRUE)

head(my_data)

year   id    V1    V2
2000   AA         
2001   AA         
2002   AA    2     2
2003   AA    3     3
2000   BB    4     4
2001   BB         
2002   BB    3     3
2003   BB        
2000   CC    2     2     
2001   CC    3     
2002   CC    3     3       
2003   CC         
2000   DD          4
2001   DD          2
2002   DD          
2003   DD          

How can I deal with the missing value by calculating the mean for the missing values?

It does not have to be the code I put here, if you have another method or way, please add it here.

Thank you.