Coming from someone who has worked a in the account servicing department of an actual bank in the US, other answers are right, this is probably a scam, the phone number on the letter is probably ringing to a fraudulent call center (these are very well managed and sound professional), and you must independently locate and dial the true contact number to US Bank. NOW. Tell them what happened. Reporting is critical. Securing your money is critical. Every piece of information you provided "the bank" when you called needs to be changed or worked around. Account numbers, passwords, usernames, card numbers get changed. Tax ID numbers get de-prioritized as an authentication mechanism even if the government won't change them.
The true bank probably won't transfer you to the branch. If the front-line call center says they will, ask the person on the phone what the branch can do that they cannot. Information is your friend. They will probably transfer you to a special department that handles these reports. Apparently Union Bank's call center transfers you to the branch then has the branch make this transfer. Maybe their front-line call center team is empowered to handle it like I was. Either way, plug your phone in; if the call takes less than 5 minutes they didn't actually do everything. 5 to 8 minutes per department is more likely, plus hold time. There's a lot of forms they're filling out.
What if that office is closed because of time differences? Go online and ask for an ATM limit increase. Start doing cash advances at local banks if your card allows it. Just get that money out of that account before it's in a fraudsters account. Keep receipts, even if the machine declines the transaction. Either way, get cash on hand while you wait for a new debit card and checks for the new account you're going to open.
What if this was fraud, you draw your US Bank account down to zero $800 at a time, and you don't close it or change passwords? Is it over? No. Then your account WILL get closed, and you will owe EVERYTHING that the fraudsters rack up (these charges can put your account terrifyingly far in the negative) from this point forward. This is called "participation in a scam" in your depository agreement, because you fell victim to it, didn't report, and the info used was voluntarily given. You will also lose any of your money that they spend.
What if US Bank really is closing your account? Then they owe you every penny you had in it. (Minus any fees allowed in the depository agreement). This closure can happen several days after the date on the warning, so being able to withdraw doesn't mean you're safe. Banks usually ship an official check shipped to the last known address they had for you.
Why would a bank within the United States close my account when it's not below the minimum balance? Probably because your non-resident alien registration from when you were in school has expired and federal law prohibits them from doing business with you now. These need renewed at least every three years. Renewing federally is not enough; the bank must be aware of the updated expiration date.
How do I find out why my account is being closed? You ask the real US Bank. They might find that it's not being closed. Good news! Follow the scam reporting procedure, open a new account (with US Bank if you want, or elsewhere) and close the old one. If it IS being closed by the bank, they'll tell you why, and they'll tell you what your next options are. Ask what can be done. Other commenters are right that bitcoin activity may have flagged it. That activity might actually be against your depository agreement. Or it set off a detection system. Or many other reasons. The bank who services your account is the only place that knows for sure.
If I offer them $500 per year will they likely keep the account opened? Otherwise I got to go to singapore open another account
Legitimate financial institutions in the United States don't work this way. If there is a legal problem with your tax status in the US, money to the bank won't solve it. Let's call the folks you've talked to "FraudBank" and the real USBank "RealBank," because until RealBank confirms, we have no reason to believe that the letter is real.
FraudBank will ask for money. Don't give it. Don't give them any further information. Gather up as much information from them as possible instead. Where to send it, for example. Then report that to RealBank. RealBank won't have a way to charge $500/year to you only. If they offer a type of account to everyone that costs $500, ask for the "Truth in Savings Act disclosures." Banks are legally required to provide these upon request. Then read them. Don't put or keep your money anywhere you don't understand.