Two U.S. officials who spoke on the condition of anonymity said the arrangement would be similar to the NATO mutual defense agreement. But crucial details remain unsettled as negotiators seek a deal that Russia will approve.

The talks come as the US pushes Ukraine to accept its plan for implementing a ceasefire.

Dec. 15 (UPI) -- British Prime Minister Keir Starmer and German Chancellor Friedrich Merz were set to meet with Ukrainian President Volodymyr Zelensky and U.S. officials in Berlin on Monday in an effort to reach a consensus on what any peace deal with Russia should look like.