Russia may have lost a third of the invasion force it sent into Ukraine as its offensive continues to struggle in the face of stiff resistance, British military intelligence has said.

In its latest assessment, the Ministry of Defence (MoD) said the Russian campaign in the east of Ukraine had “lost momentum” and was now “significantly behind schedule”.

At a meeting of Nato foreign ministers in Berlin, the foreign secretary, Liz Truss, said it was essential to maintain support for the government in Kyiv to help it “push Russia out”.

She said in a statement: “Putin must face a sustained defeat in Ukraine, Russia must be contained and such aggression must never happen again.

“Ukraine’s security must come from it being able to defend itself. Allies must support Ukraine’s move to Nato-standard equipment, immediately providing artillery, training and the required expertise.”

The Nato deputy secretary general, Mircea Geoană, said the Ukrainians were now in a position to defeat the Russians and win the war.

“The brutal invasion of Russia is losing momentum,” he told reporters.

“With significant support from allies and partners in billions of dollars, in military support, in financial support, humanitarian support, we know that with the bravery of the Ukrainian people and army and with our help, Ukraine can win this war.”

The Russians switched the focus of their offensive to the eastern Donbas region – which was already part-held by pro-Moscow separatists – after their advance on Kyiv was driven back.

However the MoD said despite small-scale initial advances, they had failed to make any substantial territorial gains over the past month while suffering “consistently high levels of attrition”.

It said the offensive was being further hampered by the loss of “critical enablers” such as bridging equipment and intelligence, surveillance and reconnaissance drones.

“Russia has now likely suffered losses of one-third of the ground combat force it committed in February,” it said.

“Russian forces are increasingly constrained by degraded enabling capabilities, continued low morale and reduced combat effectiveness.

“Many of these capabilities cannot be quickly replaced or reconstituted and are likely to continue to hinder Russian operations in Ukraine.

“Under the current conditions, Russia is unlikely to dramatically accelerate its rate of advance over the next 30 days.”