Combining an ifelse statement with shift data.table function in R

I am trying to work out how I would combine an ifelse statement with the shift function in data.table. My data looks like this:

CHR SNP        BP       KBdist   locus
1   rs2494631  2399149  30       1
1   rs4648637  2401364  2215     NA
1   rs2494627  2402499  1135     NA
1   rs11122119 6768856  4366357  NA
1   rs1844583  8383469  1614613  NA
1   rs2292242  8385059  1590     NA

and what I am trying to achieve is: "If CHR is equal to the line above, and KBdist is less than 500,000, make locus equal to the line above, else add one". Which would yield an output that looks like this:

CHR SNP        BP       KBdist   locus
1   rs2494631  2399149  30       1
1   rs4648637  2401364  2215     1
1   rs2494627  2402499  1135     1
1   rs11122119 6768856  4366357  2
1   rs1844583  8383469  1614613  3
1   rs2292242  8385059  1590     3

I know that I can use shift to access the values in the row above, for example:

DF<-DF[ , KBdist := BP - shift(BP, 1L, type="lag")]

As that is how I created one of the columns. But I don't see how you could extend it to including the ifelse statement conditions above.

Any help would be greatly appreciated.

Thanks in advance.