LONDON (Reuters) – The heads of the U.S. CIA and Britain’s spy service said in an op-ed on Saturday that “staying the course” in backing Ukraine’s fight against Russia was more important than ever and they vowed to further their cooperation there and on other challenges.

The op-ed in the Financial Times by CIA Director William Burns and Richard Moore, chief of the Secret Intelligence Service, was the first ever jointly authored by heads of their agencies.

“The partnership lies at the beating heart of the special relationship between our countries,” they wrote, noting that their services marked 75 years of partnership two years ago.

The agencies “stand together in resisting an assertive Russia and (Russian President Vladimir) Putin’s war of aggression against Ukraine,” they said.

“Staying the course (in Ukraine) is more vital than ever. Putin will not succeed in extinguishing Ukraine’s sovereignty and independence,” they said, adding their agencies would continue aiding Ukrainian intelligence.

Russian forces have been slowly advancing in eastern Ukraine, Ukrainian troops have been occupying a large swath of Russia’s Kursk region and Kyiv has been pleading for more U.S. and Western air defenses.

The spy chiefs said their agencies would keep working to thwart a “reckless campaign of sabotage across Europe by Russian intelligence” and its “cynical use of technology” to spread disinformation “to drive wedges between us.”

Russia has denied pursuing sabotage and disinformation campaigns against the U.S. and other Western countries.

Burns and Moore noted that they had reorganized their agencies to adapt to the rise of China, which they called “the principle intelligence and geopolitical challenge of the 21st Century.”

The agencies, they said, also “have exploited our intelligence channels to push hard restraint and de-escalation” in the Middle East, and are working for a truce in Gaza that could end the “appalling loss of life of Palestinian civilians” and see Hamas release hostage it seized in its Oct. 7 assault on Israel.

Burns is the chief U.S. negotiator in talks to reach a deal.

(Reporting by Jonathan Landay; Editing by David Gregorio)