Even as it engages in fierce fighting with Russia on the battlefield, Ukraine is also waging war on a different, more shadowy front: rooting out spies and collaborators in government and society who are providing crucial help to the invading forces.

While Ukrainian society as a whole has rallied to the country’s defense, Russian sympathizers are reporting the locations of Ukrainian targets like garrisons or ammunition depots, Ukraine’s officials say. Priests have sheltered Russian officers and informed on Ukrainian activists in Russian-occupied areas. One official said collaborators had removed explosives from bridges, allowing Russian troops to cross.

The issue was cast into sharp relief on Sunday night when President Volodymyr Zelensky dismissed two senior law enforcement officials, saying they had not been nearly aggressive enough in weeding out traitors. It was the first major reshuffle of his brain trust since the war began.

Hundreds of treason investigations have been opened, Mr. Zelensky said in a televised address after the dismissals, which still must be confirmed by the Parliament, underscoring the depth of a problem that can provide a critical advantage to the enemy. The threat from spies in government, churches and intelligence agencies, and from Russian-leaning citizens in the East, has plagued Ukraine for years, but has become still more acute during the war.

Mr. Zelensky specifically cited Ukraine’s security service, an unwieldy force of 27,000 personnel, the largest in Europe. Western allies believe the service has too many areas of operation, leaving it open to corruption, and prone to straying from its spy-hunting role.

“Such an array of crimes against the foundations of the national security of the state, and the connections detected between the employees of the security forces of Ukraine and the special services of Russia, pose very serious questions to the relevant leadership,” Mr. Zelensky said. One of the officials Mr. Zelensky ousted on Sunday was the head of the domestic intelligence agency, Ivan Bakanov, a childhood friend of the president. The other was Iryna Venediktova, Ukraine’s prosecutor general. While Mr. Zelensky did not accuse them of betrayal, he suggested they had turned a blind eye to traitors in sensitive positions.