"We write to urge you to comply with all American sanctions, but also to emphasize we would consider it particularly troubling if you sought to evade or undermine American statutes," the letter, which was dated Thursday and published by t-online.de, says.
The US lawmakers stressed that US sanctions were not just a government decree, but that many of them originate from federal laws passed by Congress.
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The senators also noted that sanctions were a considered and long-term US policy, and attempts to circumvent them by European countries will prompt relevant US actions.
Washington's European partners have repeatedly stated that they intended to continue to observe the terms of the deal with Iran.