The head of Britain's foreign intelligence service used a rare speech Wednesday to issue a plea to disaffected Russians to spy for the UK.

Speaking in Prague, Richard Moore appealed to Russians "wrestling with their conscience" to take a stand against Russian President Vladimir Putin's regime and offered them the opportunity to "share secrets with MI6."

Moore went on to state that "there are many Russians today who are silently appalled by the sight of their armed forces pulverizing Ukrainian cities, expelling innocent families from their homes, and kidnapping thousands of children."

"They are watching in horror as their soldiers ravage a kindred country. They know in their hearts that Putin's case for attacking a fellow Slavic nation is fraudulent," he added.

Moore said he was "optimistic" Ukraine would prevail against the Russian invasion, saying Kyiv had "recovered more territory in a month than the Russians managed to achieve in a year" and that "there appears to be little prospect of the Russian forces regaining momentum."

His comments come as Putin continues to escalate his war in Ukraine, with Russian forces mounting an aerial assault on the city of Odesa overnight into Wednesday.

Meanwhile, Russia's termination of a crucial deal that allowed the export of Ukrainian grain has further ramped up diplomatic tensions, and the aftermath of an armed uprising by the Russian mercenary group Wagner last month -- ended by a pardon for leader Yevgeny Prighozin and his departure to Belarus -- continues to raise questions for Putin.

The MI6 chief told CNN that Prighozin is alive and at liberty following the group's 24-hour mutiny against the Kremlin, and that Putin is clearly under pressure.

"You don't have a group of mercenaries advance up the motorway towards Moscow and get to within 125 kilometers of Moscow unless you have not quite predicted that was going to happen."

Moore said that Putin "didn't really fight back" against Prighozhin but instead had to cut a "humiliating deal" to bring the mutiny to an end. "He has to have realized, I am sure that something that is deeply rotten in the state of Denmark -- to quote Hamlet -- and he had to cut this deal."

He also issued a warning to African states with connections to the Wagner Group that if Prigozhin could "betray" Putin, then it will betray them in turn.

Wagner mercenaries have been present in the Central African Republic, Sudan, Libya, Mozambique, and Syria. Over the years they have developed a particularly gruesome reputation and have been linked to various human rights abuses.

A CNN investigation in April found that Wagner has been supplying Sudan's paramilitary Rapid Support Forces with missiles to aid their fight against the country's army.