ISTANBUL -- There is "absolutely no evidence" that Russia's President Vladimir Putin wants to negotiate peace in Ukraine, the head of Britain's foreign intelligence agency said Friday in an outgoing speech.

Richard Moore, chief of the Secret Intelligence Service, or MI6 as it is more commonly known, said Putin was "stringing us along."

"He seeks to impose his imperial will by all means at his disposal. But he cannot succeed," Moore said. "Bluntly, Putin has bitten off more than he can chew. He thought he was going to win an easy victory. But he - and many others - underestimated the Ukrainians."

The war has continued unabated in the three years since Russia invaded its neighbor, despite renewed U.S.-led efforts in recent months to steer Moscow and Kyiv to a settlement. Ukraine has accepted proposals for a ceasefire and a summit meeting, but Moscow has demurred.

U.S. President Donald Trump said Thursday during a state visit to the United Kingdom that Putin "has really let me down" in peace efforts.

Moore was speaking at the British consulate in Istanbul after five years as head of MI6. He leaves the post at the end of September. The agency will then get its first female chief.

During his tenure, Russia launched its full-scale invasion of Ukraine in February 2022, a war that has seen tens of thousands killed and still rages, principally in eastern Ukraine.

Moore said the invasion had strengthened Ukrainian national identity and accelerated its westward trajectory, as well as pushing Sweden and Finland to join NATO.

"Putin has sought to convince the world that Russian victory is inevitable. But he lies. He lies to the world. He lies to his people. Perhaps he even lies to himself," Moore told a news conference.

Referring to the Russian scientist Ivan Pavlov, who conditioned dogs to respond to a bell, Moore said a phone call from the Russian president was "the equivalent of Pavlov's tinkling bell inside the Kremlin, eliciting learned behavior to tell Putin whatever it is the system thinks he want to hear."

He said that Putin was "mortgaging his country's future for his own personal legacy and a distorted version of history" and the war was "accelerating this decline."

Moore, who previously served as the U.K.'s ambassador to Ankara, the Turkish capital, added that "greater powers than Russia have failed to subjugate weaker powers than Ukraine."

Analysts say Putin believes he can outlast the political commitment of Ukraine's Western partners and win a protracted war of attrition by wearing down Ukraine's smaller army with sheer weight of numbers.

Ukraine, meanwhile, is racing to expand its defense cooperation with other countries and secure billions of dollars of investment in its domestic weapons industry.

The spy chief was speaking as MI6 unveiled a dark web portal to allow potential intelligence providers to contact the service. Dubbed " Silent Courier," the secure messaging platform aims to recruit new spies for the U.K., including in Russia.

"To those men and women in Russia who have truths to share and the courage to share them, I invite you to contact MI6," Moore said.

Not just Russians but "anyone, anywhere in the world" would be able to use the portal to offer sensitive information on terrorism or "hostile intelligence activity," he said.

Head of MI6 says there's 'absolutely no evidence' Putin wants to...