Why Did Japan Freeze Iranian Assets?

In an attempt to abide by the sanctions of the United Nations that were aimed to encumber Iran from nuclear proliferation, Japan froze the assets of Iranian individuals and institutions.  This was an addition of fifteen individuals and about thirteen institutions to the list of already frozen Iranian assets as told by the representative of the Japanese government, Yasu Hisa Shiozaki who is the Chief Cabinet secretary.  Not only that but Japan is also aiming to ban any kinds of aids, grants or loans to Iran for everything but the purposes of humanitarian or development nature.  Japan, which is very poor in terms of resources, finds itself imprisoned in a tight quagmire as it has become verily difficult for the developed Asian country to cater to its energy requirements that it refers to Iran for and being in sync with the requirements of 'ethics' that its closest security ally, the United States demand from it.  Nevertheless from the recent move by Japan, it seems that the proclivity of Japan lies more in the favor of the United States more than Iran.