"Because it all comes down to (Russian President Vladimir) Putin's refusal to end this war, and to decisions on the American side that are hard to explain between friends," Zelenskiy said in a meeting with a US congressional delegation.
The meetings come amid increased tensions between Washington and Kyiv.
Ukraine alleges the speed of its Starshield military-grade satellite system, owned by SpaceX, was reduced by a US policy decision and the US has limited intelligence sharing and delayed delivery of Patriot air defence missiles.
"All of this suggests that some political decision has been made to slow down support for us," Ukrainian President Volodymyr Zelenskiy said in a video posted on Monday on Telegram.
Meanwhile, US President Donald Trump said he thought someone other than Zelenskiy should negotiate on Kyiv's behalf as it seeks to end the war with Russia.
"Maybe it's time for Zelenskiy to get somebody else to negotiate," said Trump, who on October 10 had called for Ukraine to replace the President.
Trump also told reporters that Russia has been "fine" with regards to Ukraine.
"They haven't been doing much," he said.
Zelenskiy said that on three occasions in the past week, Ukraine had not received the advance warning of an attack generally provided by its partners.
"We also know that, unfortunately, the Pentagon is delaying the delivery of missiles that our partner have already paid for through the PURL programme," he said on X.
He said one missile package was waiting for signature by US Defense Secretary Pete Hegseth.
Zelenskiy also shared details of "a horrific Russian strike" in the Kharkiv region.
"A "shahed" struck people directly - in a park near an ordinary café. Pure atrocity. Six people were killed, including children. A little girl was killed in a stroller," he said on X.
Zelenskiy said Ukraine was willing to halt strikes on Russian oil refineries for 30 days if Washington supplied Ukraine with interceptors for the Patriot anti-missile system.
He he hoped a Ukrainian delegation would meet in the coming days with US presidential envoys Steve Witkoff and Jared Kushner in Norway.
Reuters reports Witkoff and Kushner are set to travel to Norway this week for negotiations on ending the war in Ukraine, according to three people familiar with the matter.
Two of the people said they expected Zelenskiy to attend the meetings.
The people, who spoke on condition of anonymity, said the schedule is tentative and could change.
