Denmark, with a population of less than 6 million, in March established a $1 billion fund for military, civilian and business aid to Ukraine in 2023.

Frederiksen, seen as a possible contender to become new NATO chief, on Monday told Danish public radio the government planned to add another 7.5 billion crowns to the fund this year, and 10.4 billion next year.

"This major contribution will further strengthen the combat capabilities of the Armed Forces of Ukraine in the short and medium term," Zelenskiy said in a tweet. "Our strength is in unity!"

The new money is earmarked for military aid, Danmarks Radio reported.

The announcement comes ahead of visits this week by U.S. Secretary of State Antony Blinken to Denmark's fellow Nordic countries Sweden, Norway and Finland.

Jens Stoltenberg, the transatlantic military alliance's secretary-general, is due to step down in September.

Finland recently joined NATO due to Russia's invasion of Ukraine, and Sweden hopes to join by the alliance's summit in Vilnius in July.

(Reporting by Anna Ringstrom; Editing by Nick Macfie)