LONDON — European officials and politicians reacted angrily on Sunday to reports that the United States has been spying on its European Union allies, saying the claims could threaten impending talks with Washington on an important trade agreement.

The latest accusations surfaced in the online edition of the German newsmagazine Der Spiegel, which reported on Saturday that American agencies had monitored the offices of the European Union in New York and Washington. Der Spiegel said information about the spying appeared in documents obtained by Edward J. Snowden, the former American intelligence contractor, and seen in part by the magazine.

On Sunday, the online edition of the British newspaper The Guardian reported additional details about the surveillance program. The newspaper said that one document it had obtained listed 38 embassies and diplomatic missions in Washington and New York, describing them as “targets.” It detailed a broad range of spying methods used against one, including bugs implanted in electronic communications gear and the collection of transmissions using specialized antennas.

The list of targets included the European Union’s missions and the French, Italian and Greek Embassies, as well as those of several other American allies, including India, Japan, Mexico, South Korea and Turkey, The Guardian reported.