While India has continued to trade with Iran during previous such sanctions, it has had to cut oil imports as the sanctions (that also prohibit transactions by foreign financial institutions with the Central Bank of Iran) choke off banking channels and insurance cover for tankers that carry oil.

from Times of India https://timesofindia.indiatimes.com/business/india-business/why-cant-india-ignore-us-sanctions-on-iran/articleshow/69056991.cms