Volodymyr Zelensky, the Ukrainian president, said the country had opened 651 treason investigations into employees of the country’s law enforcement agencies and others who were suspected of working with Russia.

Russian sympathizers are reporting the locations of Ukrainian targets like garrisons or ammunition depots, officials say. Priests have sheltered Russian officers and informed on Ukrainian activists in Russian-occupied areas. One official said collaborators had even removed explosives from bridges, allowing Russian troops to cross.

Ukraine’s shadow war against Russian collaborators came into sharp relief on Sunday, when Zelensky dismissed two senior law enforcement officials. He did not accuse them of betrayal, but suggested that they had turned a blind eye to traitors in sensitive positions.

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Zelensky specifically cited Ukraine’s security service, an unwieldy force of 27,000 personnel, the largest in Europe. Many intelligence chiefs graduated from K.G.B. schools, and Western allies say that the service has too many areas of operation, leaving it open to corruption and prone to straying from its spy-hunting role.

Context: In Crimea, which Moscow annexed in 2014, and in Ukraine’s east, where fighting has recently intensified, deep cultural and historical ties with Russia have translated to pockets of support for Moscow. The threat has plagued Ukraine for years but has become more acute during the war.

The E.U.: Foreign ministers added gold to the list of banned Russian imports and approved 500 million euros to reimburse member states for weapons sent to Ukraine. That brings the bloc’s total spending on Ukrainian military aid to 2.5 billion euros.