Ukraine and its allies agreed Tuesday to a set of principles for rebuilding the war-torn country, including the need for broad reforms to boost transparency and root out corruption.

Wrapping up a two-day conference in the southern Swiss city of Lugano, leaders from some 40 countries signed the Lugano Declaration committing to support Ukraine through a likely long and expensive recovery.

Ukrainian President Volodymyr Zelensky warned from Kyiv that the work ahead was "colossal", and the duty of the "whole democratic world".

His prime minister, Denys Shmyhal, who led a large delegation to Lugano, cautioned that recovery from the massive destruction wrought since Russia's full-scale invasion just four months ago would cost at least $750 billion.

He said the adoption of the declaration and of a set of seven founding principles for Ukraine's reconstruction "gives great hope".

"We shall be victorious, we will renew our country," he told reporters. "We have to make everything that was destroyed better than it was."

Swiss President Iganzio Cassis, who co-hosted the conference, hailed the declaration as a "key first step on the long road of Ukraine's recovery".

"Our work prepares for the time after the war even as the war is still raging," he told the closing ceremony following a minute of silence for that war's many victims.

Among the principles agreed upon Tuesday was that Ukraine itself must be in the driving seat on how to rebuild, and also that the recovery process must go hand-in-hand with far-reaching reforms.

"The rule of law must be systematically strengthened and corruption eradicated," the document said.

With billions of dollars in aid and assistance flowing into Ukraine, lingering concerns about widespread corruption have driven calls Kyiv to do more to ensure transparency and accountability.

The former Soviet state has long been ranked among the world's most corrupt countries by Transparency International. In Europe, only Russia and Azerbaijan ranked worse.

Ukraine President Volodymyr Zelensky addressed the Ukraine Recovery Conference on Monday
Ukraine President Volodymyr Zelensky addressed the Ukraine Recovery Conference on Monday AFP / Fabrice COFFRINI

Shmyhal insisted Tuesday that Ukraine had already taken great strides to fix the problem, including by broad digitalisation of public services and the awarding of contracts in sectors like construction, to reduce "human interaction" and the possibilities for corrupt transactions.

The goal, he said, is "not to fight corruption, but make corruption impossible."

As for who will pay for the towering costs, Shmyhal suggested much of this amount could be covered using seized Russian assets. He pointed out that such assets frozen by Ukraine's partners so far amounted to $300-500 billion.

"Unprovoked aggression should be paid by the aggressor," he said. "Russia should pay for this."

At his side, the Swiss president, whose country has long been a choice destination for Russian oligarchs to invest and stash away their fortunes, stressed the importance of respecting property rights and the rule of law.

Shmyhal on Monday laid out the government's three-phase reconstruction plan, focused on the immediate needs of those affected by the war, followed by the financing of thousands of longer-term reconstruction projects, and ultimately on transforming Ukraine into a European, green and digital country.

To push the message, a number of ministers, as well as First Lady Olena Zelenska, also spoke Monday to lay out the massive reconstruction needs, as well as their vision for a new Ukraine.

The Ukrainians have proposed that allied countries "adopt" specific regions of Ukraine, and lead the recovery there to render it more efficient.

Britain has proposed taking on the Kyiv region, while France would concentrate on the heavily-hit Chernihiv region. Australia and Denmark are also among countries that have voiced interest in leading specific reconstruction efforts.

"We understand that this is for the long-haul, and we are ready," high-level French diplomat Francois Delattre told AFP.

Lugano was seen as a first step towards the rebuilding of Ukraine, and there are already several follow-up conferences planned, with one led by the EU in a few months.

London has agreed to host a Ukraine Recovery Conference next year, while Germany has said it can host the 2024 edition.