BBC - Donald Trump has blocked numerous sanctions on Russia two days before they were due to take effect, as a row with his Ukrainian counterpart Volodymyr Zelensky deepens.
The sanctions were passed by the US Congress and signed into law by Trump in September.
But the US president issued an executive order on Friday night saying that suspending some was in the US "national interest", prompting a backlash from US lawmakers.
Relations between Washington and Kyiv frayed last week, after the US struck a controversial deal to import Russian diesel. Ukraine then accused the Trump administration of withdrawing key military support.
Trump and Zelensky have since exchanged sharp words about one another. Ukraine's president declared it was nice to be among "real leaders" while addressing an EU summit, after Trump said Zelensky was "more difficult to deal with" than Russian President Vladimir Putin.
The new US sanctions law took aim at Russia's economy, giving Trump broad powers to levy tariffs of up to 100% on imports from the top five purchasers of Russian oil and gas – most significantly China and India.
It also included sanctions against senior Russian figures, including Putin, as well as Russian financial institutions and Moscow's so-called "shadow fleet" of vessels used to circumvent existing embargoes.
Many of these provisions have now been waived by Trump.
Democrats including the Virginia Congressman Don Beyer have criticised the move, accusing Trump of capitulating to Moscow and turning his back on Kyiv and Zelensky.
"Trump's latest surrender to Putin again betrays our Ukrainian partners and defies clear congressional intent of legislation designed to punish Russia for its illegal war of aggression," Beyer wrote on X.
The move is likely to draw further anger from Ukraine, where Zelensky has long stressed the need for stringent sanctions to limit funding Russia can use in its war effort.
Trump was initially criticised by Zelensky and Ukraine's European allies for temporarily suspending some US sanctions to import Russian diesel in a bid to bring down US energy prices, which have skyrocketed during the Iran war.
Trump has spent months grappling with the political consequences of the Middle East conflict, which has sent fuel costs soaring ahead of the US midterm elections. Rising inflation has soured Trump's standing with the American public.
Zelensky suggested the deal was tantamount to the US investing in Russia's war effort.
Both nations have been intensively striking one another's energy infrastructure. Kyiv says this is to limit revenue for Russia's war machine, while Moscow claims it only targets facilities used by the Ukrainian military.
Zelensky initially expressed surprise at Trump's announcement, and said Kyiv would stop the strikes if Russia did the same. After three days which saw several fresh attacks from Russia, Kyiv resumed its strikes on Russian oil refineries.
This prompted scathing condemnation from Trump, who called the strikes a "disgrace". Zelensky replied that he was "only standing up for Ukraine's interests".
Concerns that Trump is siding with Russia have been heightened by disruption to a US-provided satellite system used by Ukraine.
Last week Starlink suddenly and unexpectedly restricted the speed at which drones could fly while still maintaining internet connection. This disrupted some battlefield missions, Ukraine said, and hampered Kyiv's ability to protect itself from incoming Russian attacks.
During the slowdown, Russia launched several deadly strikes on Ukraine, with residents and the government saying these came with no prior warning.
Zelensky said on Friday that Starlink speeds had still not been fully restored.