Retired Lieutenant General Keith Kellogg, who is set to serve as Trump's special envoy for Ukraine and Russia, had initially planned a mission to Kyiv to meet with Ukrainian leaders in early January, Reuters reported last month.  

His team had also set up meetings with officials in other European capitals, including Rome and Paris. 

But the trip, which would have been the first visit to Kyiv by members of the incoming administration since the November 5 election, has now been pushed back to an unspecified date, four sources said.  

The reason for the postponement remains unclear.  

Trump repeatedly said on his campaign trail that he could solve the war in Ukraine within 24 hours of taking office but has since made little progress, despite resolving the conflict remaining a key priority of his administration. 

Neither Kellogg nor a representative for the Ukrainian embassy in Washington have so far responded to requests for comment.