European leaders including French President Emmanuel Macron have been pushing for greater EU defence cooperation since Trump's election win in November, amid questions over the incoming US administration's commitment to transatlantic security.

Trump has long been critical of the NATO military alliance, suggesting Washington would not come to the aid of member states that failed to meet their financial obligations to the group.

"We decided to increase our efforts to make our joint European defence better," German Defence Minister Boris Pistorius said at a press conference with Ukrainian President Volodymyr Zelensky.

He said this was due to the "challenges and threats" facing Europe, as well as the "change of the administration in Washington in the days to come."

Pistorius was speaking after meeting counterparts from France, Italy, Poland, and the UK in Warsaw on Monday, a week before Trump enters the White House.

Berlin has been the second largest provider of aid to Ukraine after the United States but there have been frequent debates within Germany over the extent of that support.

The Spiegel weekly reported last week that the German government was at odds over whether to approve three billion euros ($3.1 billion) of new military aid to Ukraine.

"This is just a fiscal problem of the government we have to solve and we are working on that," Pistorius said Tuesday, adding he was "optimistic" it would be approved.