Combine two rasters with different spatial coverage

Dear list,

  I am trying to combine two rasters. One raster has global coverage, while other covers only Amazon.

  Let me show in pratice. First I create an emtpy structure with the intended domain (global) and resolution (0.5 degree):

library(raster)
glb<-raster(nrows=360,ncols=720,xmn=-180,xmx=180,ymn=-90,ymx=90)
glb[]<-NA
glb

class       : RasterLayer 
dimensions  : 360, 720, 259200  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
values      : in memory
min value   : NA 
max value   : NA 

  On the other hand, I load a Amazon deforestation file which contains values between 0 and 1:

amz<-raster("/home/data/amz.img")
amz

class       : RasterLayer
dimensions  : 47, 65, 3055  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -74, -41.5, -17.5, 6  (xmin, xmax, ymin, ymax)
coord. ref. : NA
values      : /home/data/amz.img
min value   : 0
max value   : 1

  Next, I apply the same projection:

projection(amz) <- projection(glb)
amz

class       : RasterLayer 
dimensions  : 47, 65, 3055  (nrow, ncol, ncell)
resolution  : 0.5, 0.5  (x, y)
extent      : -74, -41.5, -17.5, 6  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0 
values      : /home/data/amz.img 
min value   : 0 
max value   : 1 

  Last, after trying to replace NA values in the global raster with the values of the amazon object,

result <- cover(glb,amz)

a erratic stripe across the globe is produced, like showed in this image: http://img269.imageshack.us/img269/1328/errorau.png

  Does anyone have a suggestion to solve this problem?

  Thanks in advance,
  Thiago.