Hackers have broken into real news websites and posted fake stories stirring up anti-Nato sentiment, a cyber-security firm has warned.
The disinformation campaign, nicknamed "ghostwriter", has been ongoing since 2017, according to FireEye researchers.
It is designed to "chip away" at support for Nato in Lithuania, Latvia, and Poland, they said.
While the false stories are "aligned with Russian security interests", it is not known who is behind the attack.
The disinformation campaign uses "falsified news articles, quotes, correspondence and other documents designed to appear as coming from military officials and political figures in the target countries," FireEye said.
In some cases, false news stories were posted on real news websites without permission.
News websites typically use a content management system (CMS) to handle the large number of articles published.
The attackers apparently gained access to the CMS of the target website and replaced old articles with their own content, or posted entirely new false articles.
They would try to spread the fake stories on social media before they were taken down.
In one example from last year, a Lithuanian news site published a fake article claiming that German soldiers had desecrated a Jewish cemetery.
In another, a fake message was posted to the Polish War Studies Academy website, claiming to be from the organisation's commander. It called for troops to fight against "the American occupation".
Those attacks were complemented by other methods - such as opinion pieces and blog posts written by non-existent "journalists", and fake emails designed to look like they had come from government officials, military officers or journalists.
Some of the attacks had previously been reported by national authorities.
But FireEye's report collected the various individual attacks into what it said was a "broader influence campaign".
There have been rising tensions between Russia and several Nato members in recent years.
Poland has sought to have the United States forge a permanent military base in the country, while Russia says the arrival of US troops on rotation there in recent years is a threat to its security.
Lithuania, once a part of the Soviet Union, has said it is concerned about a potential threat from Russia, and announced plans to build a fence around the Russian enclave of Kaliningrad.
Latvia has a large and vocal ethnic Russian population. A pro-Russia party won the most votes in the 2018 election, though it did not enter government.
Latvia, Lithuania and Poland are all members of Nato, which was founded after World War Two to balance the threat of communist expansion from the Soviet Union into Europe.