Croatia is expected to join the EU in July and the US expects that to happen, said Reeker, who labelled the LB issue "a complex dispute" but said it was "not a US issue."

"We don't have a role in mediating or any role to play other then encouraging both countries to work together," he said.

Reeker said it was good that the two countries appointed experts to tackle the issue, who will hold their next meeting on 22 January.

"I think that's an important sign and they should clearly use this opportunity to engage fully and come to whatever arrangements, conclusions are necessary to move forward on that."

Reeker deems progress in the enlargement process important for the EU as well as Slovenia and Croatia, which "share the past and the future" as neighbours.