European Union foreign policy chief Catherine Ashton expressed concern on Thursday after Russia's lower house passed a law broadening the definition of treason.
"The new law would expand the scope for prosecution of and reduce the burden of proof for charges of treason and espionage," her office said in a statement.
"The abstract definition of treason contained in the law will make it difficult to apply in a fair manner. It also potentially penalises contacts with foreign nationals with up to 20 years in prison."
Human rights activists have attacked the bill passed on Tuesday as a further attempt to curb opposition to President Vladimir Putin. They say the measures could criminalise sharing information with organisations such as Amnesty International or even appealing to the European Court of Human Rights.
The statement said the new law follows a number of recent legislative and judicial developments in Russia that, taken together, "would limit the space for civil society development, and increase the scope for intimidation."
"We will be monitoring the implementation of this law closely," the statement added.
The bill is likely to be swiftly passed by the upper house of parliament and signed into law by Putin.
It follows legislation that brands advocacy groups with foreign funding as "foreign agents", criminalises slander and blacklists websites unfavourable to the government -- all introduced within months of Putin's return to the Kremlin in May for a third presidential term.