space not getting released

Hello: We have a Linux 2.6.18-194.el5 #1 SMP Mon Mar 29 22:10:29 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux system. I deleted a bunch of large files but the space is not getting released to the file system. If I do a du -sk . on the file system it gives me 180G but when I do a df -k it shows 300G used space. I am not able to create new files in that file system because of this. What can I do. Any help will be greatly appreciated. Thx. Singh

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