Merz has convened a series of virtual meetings for Wednesday in an attempt to have the voice of European and Ukraine's leaders heard before the summit later this week in Alaska, where President Donald Trump and Russian President Vladimir Putin are expected to discuss a path towards ending Moscow's war in Ukraine.
Zelenskiy and the leaders of European countries have been sidelined from that summit.
Zelenskiy is due to meet with European leaders first, in preparation for a virtual call with Trump and Vice President JD Vance about an hour later.
A call between leaders of countries involved in the "coalition of the willing" - those who are prepared to help police any future peace agreement between Moscow and Kyiv - will take place last.
On Wednesday, Merz pledged to help Ukraine develop its own long-range missile systems that would be free of any Western-imposed limitations on their use and targets as the Kyiv government fights to repel Russia's invasion.
The Ukrainian leader on Wednesday reiterated his doubt that Putin would negotiate in good faith to end the war.
Writing on his official Telegram channel, Zelenskiy said there was "currently no sign that the Russians are preparing to end the war".
He urged Ukraine's partners in the United States and Europe to coordinate efforts and "force Russia to peace".
"Pressure must be applied on Russia for an honest peace. We must take the experience of Ukraine and our partners to prevent deception by Russia," Zelenskiy said.
Trump has said he wants to see whether Putin is serious about ending the war, now in its fourth year, describing Friday's summit as "a feel-out meeting" where he can assess the Russian leader's intentions.
Yet Trump has disappointed allies in Europe by saying Ukraine will have to give up some Russian-held territory.
He has also said Russia must accept land swaps, although it was unclear what Putin might be expected to surrender.
European allies have pushed for Ukraine's involvement in any peace talks, fearful that discussions that exclude Kyiv could otherwise favour Moscow.
Trump on Monday ducked repeated chances to say that he would push for Zelenskiy to take part in his discussions with Putin, and was dismissive of Zelenskiy and his need to be part of an effort to seek peace.
Trump said that following Friday's summit, a meeting between the Russian and Ukrainian leaders could be arranged, or that it could also be a meeting with "Putin and Zelenskiy and me".
The Europeans and Ukraine are wary that Putin, who has waged the biggest land war in Europe since 1945 and used Russia's energy might to try to intimidate the European Union, might secure favourable concessions and set the outlines of a peace deal without them.
The overarching fear of many European countries is that Putin will set his sights on one of them next if he wins in Ukraine.
Zelenskiy said Putin wanted Ukraine to withdraw from the remaining 30 per cent of the Donetsk region that it still controls as part of a ceasefire deal, a proposal the leader categorically rejected.
Zelenskiy reiterated that Ukraine would not give up any territory it controls, saying that would be unconstitutional and would serve only as a springboard for a future Russian invasion.
He said diplomatic discussions led by the US focusing on ending the war have not addressed key Ukrainian demands, including security guarantees to prevent future Russian aggression and including Europe in negotiations.
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