REUTERS/Jonathan ErnstThere is active work under way to reconcile the territorial issue of Donetsk at U.S.-mediated talks to end Russia's war in ‌Ukraine, U.S. Secretary of State Marco Rubio said on Wednesday, describing the disagreement as a key remaining issue that is "very difficult" to resolve. The top U.S. diplomat said there might be a U.S. presence in the follow-up Ukraine talks, but President Donald Trump's top envoys Steve Witkoff and Jared Kushner, who had taken part in the previous round of talks last weekend in Abu Dhabi, will not be participating. Rubio was also asked ‍if security guarantees for Ukraine were agreed to by the U.S. and ‌Ukrainian sides. And of course, any security ‌guarantees would come into play after the conflict would end," ‍Rubio said. The Financial Times reported on Tuesday that Washington told Ukraine it must sign on to a peace deal with Russia to get U.S. security guarantees.