"Let's be in no doubt. Our world is more dangerous and contested now than it has been for decades," Blaise Metreweli, the first woman to lead the MI6 Secret Intelligence Service (SIS), warned.

"Conflict is evolving and trust eroding, just as new technologies spur both competition and dependence," she said.

In her maiden speech, the new head of Britain's MI6 foreign intelligence service highlighted the "threat" posed by an "aggressive, expansionist and revisionist" Russia.

In its war against Ukraine, Russian President Vladimir Putin "is dragging out negotiations and shifting the cost of war onto his own population", she said.

"Russia is testing us in the grey zone with tactics that are just below the threshold of war," she added.

Metreweli highlighted tactics by Moscow to "bully, fearmonger and manipulate" through cyberattacks on critical infrastructure, drones buzzing around European airports, aggressive activity on the seas and state-sponsored arson.

"Across the globe, we are now confronting not one single danger, but an interlocking web of security challenges -- military, technological, social, ethical even -- each shaping the other in complex ways," she said.

"We are now operating in a space between peace and war."

And Metreweli warned that "our world is being actively remade, with profound implications for national and international security.

"Institutions which were designed in the ashes of the Second World War are being challenged."

Metreweli was appointed in June as the 18th head of the service. The MI6 chief is the only publicly named member of the organisation and reports directly to the foreign minister.

She warned of the increasingly complex nature of global threats, adding the "front line is everywhere" as a result of cyber disruption, hybrid warfare, "terrorism and information manipulation".

- 'National resilience' -

The new head of Britain's armed forces, Richard Knighton, meanwhile was Monday to call for "national resilience" in another speech at the Royal United Services Institute (RUSI), a think tank specialising in defence.

"The situation is more dangerous than I have known during my career and the response requires more than simply strengthening our armed forces," the chief of defence staff will say, according to a Ministry of Defence (MoD) statement.

"A new era for defence doesn't just mean our military and government stepping up -- as we are -- it means our whole nation stepping up."

Knighton will announce pound50 million ($67 million) in funding for new "Defence Technical Excellence Colleges" to help defence employers train up staff.

The speeches come as Prime Minister Keir Starmer was due in Berlin later Monday for talks with Ukrainian President Volodymyr Zelensky and European leaders on how to end Moscow's nearly four-year invasion.

Britain has repeatedly warned of the threat from Russia, recently raising the alarm after the government said a Russian military ship was sighted near British waters.

The MoD has just launched a new organisation -- the Military Intelligence Services -- to unify intelligence gathering and sharing efforts undertaken by the army, navy and air force.

"The announcement comes amid escalating threats to the UK, as adversaries intensify cyber-attacks, disrupt satellites, threaten global shipping lanes, and spread disinformation," the MoD said on Friday.