You don’t give your enemy ANY hint as to your capabilities or the means to defeat them; hence, you do NOT publish encryption algorithms. The NSA doesn’t publish military grade encryption for that very reason, not because they the algorithms are weak in any way. In fact, AES is pathetically weak in comparison to military grade encryption.
Like any encryption algorithm AES is not perfect: http://eprint.iacr.org/2009/374 but you can add more rounds: http://security-architect.com/does-size-matter-aes-128-bit-encryption-is-probably-good-enough/ to add strength. As the latter link mentions AES was designed and selected to be both strong and efficient. Therefore it is weaker than it could be without the efficiency requirement.
I am sure there are classified encryption algorithms. But I consider those to be relics of a bygone era. Like this now declassified: https://www.gwern.net/docs/1955-nash In the 50's we classified many things that are common knowledge now. If we still have classified algorithms, how do we know that Snowden didn't send those to the Chinese and Russians?