President Barack Obama reportedly told Russian President Vladimir Putin in October that directly interfering with the U.S. election could result in an "armed conflict."

According to NBC News, Obama opted to use the red phone to contact Moscow directly, a communication system that dates back to the Cold War. It's not an actual telephone, but instead sends a secure email message between the two countries.

"International law, including the law for armed conflict, applies to actions in cyberspace," part of the Oct. 31 message read, according to NBC. "We will hold Russia to those standards."

One of Obama's senior advisers reportedly told the president months ago that he should use the phrase "act of war" when it came to Russia's alleged meddling in the presidential election. Obama declined to use that term when the two leaders met at the September G-20 summit in China, instead telling Putin he should cease any election-hacking activities.

NBC connects Russia's alleged hacking with the release of stolen emails from Democrats, although WikiLeaks — the organization responsible for making much of the material public — denied Russia was behind the theft.

Obama's use of the red phone was the first time he had used the system during his presidency, NBC notes.