Can the IRS levy 401k and IRA for back taxes owed?

401k and IRAs are "Assets", the IRS can levy assets and seize them if you do not resolve your tax matters.

You can resolve your back taxes through different avenues, negotiated payment plans and negotiated tax settlements.

Added:

If you have unresolved tax debt, IRS is authorized to seize any property and assets, even they are held by someone else. So it means, they can levy your wages, retirement accounts, dividends, bank accounts, licenses, rental income, accounts receivables, the cash loan value of your life insurance, or commissions. Usually levy is the last tool that IRS would use to get back taxes. So if you get a letter called "Final Notice of Intent to Levy", you should be careful.