WASHINGTON—The Trump administration is maintaining open the probability of extending the New Start arms regulate treaty, but the determination will be motivated by 3 aspects, including regardless of whether China agrees to be a part of the arms regulate process, a  U.S. formal explained Tuesday.

A day just after a round of Russia-U.S. arms regulate talks in Vienna, the formal explained President Trump could agree to increase the 2011 accord, which is owing to expire in February, if the events make progress towards a new agreement that has difficult verification measures,…