How to grow a filesystem sitting on a LUKS volume with it remains open?

Issue

• I need to grow a filesystem sitting on a LUKS encrypted volume whose backing device is a logical volume luks_lv sitting on a volume group luks_vg as shown below:

# df
Filesystem           1K-blocks     Used Available Use% Mounted on
/dev/dm-3              6190656   141304   5735000   3% /luks                             <<<<

# lsblk
NAME                                                   MAJ:MIN RM  SIZE RO TYPE  MOUNTPOINT
sdc                                                      8:32   0   10G  0 disk  
└─sdc1                                                   8:33   0   10G  0 part  
  └─luks_vg-luks_lv (dm-2)                             253:2    0    6G  0 lvm   
    └─luks-a9b47d2a-2daf-4a86-83cf-784ee707f294 (dm-3) 253:3    0    6G  0 crypt /luks   <<<<

# vgs
  VG       #PV #LV #SN Attr   VSize  VFree
  luks_vg    1   1   0 wz--n- 10.00g 4.00g                                               <<<<

# lvs
  LV      VG       Attr       LSize  Pool Origin Data%  Move Log Cpy%Sync Convert
  luks_lv luks_vg  -wi-ao----  6.00g                                                     <<<<
# 

• I ran resize2fs to resize the filesystem after extending the logical volume on which the filesystem was sitting. However it said 'Nothing to do!' as shown below:

# lvextend luks_vg/luks_lv -L +1G
  Extending logical volume luks_lv to 7.00 GiB
  Logical volume luks_lv successfully resized

# resize2fs /dev/mapper/luks-a9b47d2a-2daf-4a86-83cf-784ee707f294 
resize2fs 1.41.12 (17-May-2010)
The filesystem is already 1572352 blocks long.  Nothing to do!

• How to grow a filesystem sitting on a LUKS volume with it remains open?