In 2013, Norway and the USA signed an agreement concerning the reciprocal exchange of information and the implementation of FATCA. The agreement binds Norway to have rules in place that require Norwegian financial institutions to identify customers who are US citizens and report information concerning their financial accounts. The information is sent by the Norwegian tax authorities to the US tax authorities annually. The rules were introduced because, by virtue of their citizenship, US citizens are liable to pay tax to the USA and are obliged to submit a US tax return.
Norwegian tax authorities are unable to answer questions concerning the obligation to submit a US tax return or US tax rules. Questions linked to the US tax return and US tax rules must be addressed to the US authorities. The US tax authorities have schemes for voluntary correction which are aimed at people who have not declared financial assets abroad to the US tax authorities. Read more about these schemes.
As a US citizen, you must state your US identification number to Norwegian financial institutions when asked to do so, and when you submit your tax return and other documents to the US tax authorities. Read more about US identification numbers and how to apply for one.