Washington DC - President Donald Trump said Thursday his Alaska summit with Russia's Vladimir Putin could fail but was merely a prelude to a second, three-way meeting where the substantive dealmaking over the Ukraine war would happen.

Trump and his Russian counterpart will hold talks aimed at settling the Ukraine conflict at their landmark summit at a US air base outside Anchorage on Friday.

Ukrainian President Volodymyr Zelensky is not scheduled to take part, but Trump has suggested there might be a second meeting involving both leaders if the first was successful.

"This meeting sets up the second meeting. The second meeting is going to be very, very important, because that's going to be a meeting where they make a deal," Trump told Fox News Radio.

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"And I don't want to use the word 'divvy' things up. But you know, to a certain extent, it's not a bad term, okay?"

A stepped-up Russian offensive and Zelensky's exclusion from Friday's meeting have heightened fears in Europe that Trump and Putin could strike a deal that forces painful concessions on Ukraine.

The US leader initially said there would be some "land swapping going on," but appeared to have walked that back after speaking with European leaders on Wednesday.

But his remarks to Fox News Radio suggested he had not taken some kind of exchange of territory off the table.

He added that he saw a "25% chance that this meeting will not be a successful meeting."