At 1 p.m. ET, the Twitter accounts of both companies reported that the Chinese military had fired a missile on a U.S. warship, sparking a full-blown battle. They also claimed the U.S. Federal Reserve was to make an emergency announcement about interest rates.
Within minutes, The New York Post erased most of the messages on Twitter. UPI's tweets, however, remained up.
Hackers have been known to hijack news organizations' Twitter accounts to post fake news from time to time, as the Syrian Electronic Army has done to CNN and others. When Fox News' Twitter feed was hacked in July 2011, the company tweeted that President Obama had been assassinated.
The New York Post confirmed via Twitter that its account had been hacked. UPI did not respond to a request for comment.
UPI's main Twitter account, for instance, claimed that Pope Francis had declared the start of the third World War.
Meanwhile, the New York Post business section's Twitter account said the Federal Reserve was moving -- ridiculously -- into negative interest rates.
The stock markets were unmoved by the fake news reports.
"Give the hackers this much credit, the George Washington is our permanent Pacific based carrier," tweeted John Noonan, spokesman for House Armed Services committee. "They at least did their homework."