Can the US Government seize a bank account without a warrant?

The IRS, at least, can. The IRS is specifically allowed to issue what's called a "levy" against a bank account without the need to go to court (there are other procedures that must be followed, but technically there's no warrant issued; it's a purely administrative rather than judicial process).

This may technically not qualify as a "seizure" of the account, since the levy amounts to, basically, a draft for a specific amount (as if the IRS was "writing a check" against your account), rather than the entire contents of the account. If you have more money in the account than the levy is for, the remainder will be untouched (except that banks are allowed to charge you fees for processing a levy).