At an international conference in Rome, Merz offered a message "to Washington DC and to President Donald Trump: stay with us and stay with the Europeans, we are on the same page and we are looking for a stable political order in this world".
He added a message to Russian President Vladimir Putin, saying: "We will not give up."
Merz was speaking at the Ukraine Recovery Conference, a two-day gathering intended to mobilise political and private sector support for Ukraine's reconstruction, even as the war with Russia rages.
US support for Kyiv has come into question since Trump took office in January, as the president made overtures to Moscow in an attempt to strike a peace deal.
However, this week Trump said his Russian counterpart Vladimir Putin was talking "a lot of bullshit" on Ukraine.
Trump's criticism came a day after he said he would send more weapons to Ukraine, in a reversal of Washington's announcement last week that it was halting some arms shipments.
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Merz said the EU's support for Kyiv was "unwavering", and noted it was preparing a new package of sanctions against Russia.
"There is only one member state missing to agree on that. I urgently ask Slovakia and its prime minister to give up the resistance," he said.
Slovak Prime Minister Robert Fico is known for his pro-Russian stance and was the only EU leader to attend Moscow's World War II commemorations in May, where he met Putin at the Kremlin.
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