American officials said Monday that a deal with Ukrainian leaders to end its war with Russia was nearly complete, even as territorial disputes remain unresolved and a strong security guarantee from the United States and European countries remains a sticking point.

President Donald Trump’s special envoy Steve Witkoff and his son-in-law Jared Kushner on Monday joined negotiations with Ukrainian President Volodymyr Zelenskyy in Berlin, which began Sunday.

U.S. officials in a call with reporters said that Zelenskyy, who on Sunday agreed to drop his goal of joining NATO, insists on so-called Article Five-like security protections as part of a peace deal.

Article Five of the NATO treaty states that an attack on one member nation is interpreted as an attack against all.

American officials said any such assurance from the U.S. would require congressional approval.