Liz Truss will warn the West must “never again” allow aggression like Vladimir Putin’s “to grow unchecked” – as the Ukraine invasion enters its third week.

The foreign secretary will use a speech at the Atlantic Council think tank in Washington DC on Thursday – the 15th day of the Ukraine invasion – to call for an end to “the era of complacency”.

She is expected to say: “How we respond today will set the pattern for this new era. We must start with the principle that the only thing aggressors understand is strength, and we must start by working together to stop Putin’s offensive in Ukraine.

“We must rise to this moment. We must pledge that never again will we allow such aggression to grow unchecked. That means acting now.

“It means being tough – because we know that the costs will only rise if we don’t.

“The public understands the gravity of this moment. They see the terrible suffering caused by this pre-planned, barbaric and illegal invasion against a European democracy and they recognise that the world has changed.”

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0:45

‘Direct confrontation’ if no-fly zone created

Ukraine invasion ‘paradigm shift on scale of 9/11’

Ms Truss will set out three areas where the UK is urging stronger action, including a need to end dependence on hostile and authoritarian states, including Russian energy.

She will also say there needs to be more NATO defence spending and strengthening and the UK must build stronger alliances with allies focused on trade, investment and security, including with countries not historically aligned to the UK.

Mr Putin has launched a “full-frontal assault” not just on Ukraine but also on “the very foundation of our societies and the rules by which we coexist – sovereignty, democracy, the UN Charter”, she will say.

She will add that Vladimir Putin has “shattered the architecture of global security” and the Ukraine invasion is “a paradigm shift on the scale of 9/11”.

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3:40

Zelenskyy warns ‘millions could die’

‘Can’t let authoritarians get dangerous message’

“How we respond today will set the pattern for this new era,” she is set to say.

“If we let Putin’s expansionism go unchallenged it would send a dangerous message to would-be aggressors and authoritarians around the world. We can’t allow that to happen.

“We must start with the principle that the only thing aggressors understand is strength… and we must start by working together to stop Putin’s offensive in Ukraine.”

More sanctions, weapons and aid needed

She will also call on the world to “ramp up the global pressure on Putin”, with more sanctions so the Kremlin cannot access funds, trade or travel, and more defensive weapons as well as stronger responses to the humanitarian emergency.

And she will call for countries to work together “to ensure justice is done” at the International Criminal Court.

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“We will do all of this. And we will shape this new era for global security,” she is set to conclude.

The foreign secretary has been in the US since Wednesday when she met US Secretary of State Antony Blinken and they reiterated to the world the US and UK would not be imposing a no-fly zone over Ukraine.