U.S. and Russian negotiators signaled progress Tuesday in talks on a possible replacement to a nuclear arms reduction treaty due to expire next February. But there are significant hurdles ahead — including China’s opposition to being included in the talks.At issue is the 2010 New Strategic Arms Reduction Treaty, or START, restricting the number of deployed nuclear warheads held by the U.S. and Russia, the world’s two biggest nuclear powers.  U.S. negotiator Marshall Billingslea says working group discussions may take place in late July or early August, paving the way for a possible second round of talks in Vienna.  “We did indeed hold productive talks with Russia. Indeed, the talks were so productive that we found enough common ground to warrant the establishment of several technical working groups to dive further into the details of what a future trilateral arms control agreement should look like,” Billingslea said.But there are major sticking points moving forward. Washington wants any new deal to subject China to restrictions — and include all nuclear weapons, not just strategic weapons.  Beijing, with an estimated fraction of the U.S. and Russian arsenal, has repeatedly refused to join the talks. The differences between Washington and Beijing were highlighted this week in clashing Twitter postings and official comments by the two sides.  For its part, Russia says other nuclear powers, including France and Britain, should join future talks, but on a voluntary basis.  Heading the Russian delegation, Deputy Foreign Minister Sergei Ryabkov also noted progress in Vienna, according to Russia’s TASS news agency, but also that “substantial differences” remained.  The discussions in the Austrian capital are the first between Moscow and Washington on their nuclear arsenals after more than a year’s break.  President Donald Trump has withdrawn from several U.S. treaties with Russia, including those on overflights and intermediate-range nuclear forces.  The New START treaty can be extended another five years, if both sides agree. Experts say that could pave the way for a wider-ranging and more stringent deal. Without the treaty, Washington and Moscow could be left without any significant limits on their nuclear weapons for the first time in decades.  
…