The United States and Russia have resolved a major hurdle in their negotiations to dispose of tons of excess plutonium, announcing an agreement on a severe liability issue that has stymied the program for years. The two signed a protocol that provides a framework for dealing with the issue of liability, the Energy Department announced Friday. However, other issues remain to be worked out, including details of how Russia is going to dispose of 34 metric tons of plutonium from its weapons stockpile under the agreement. At the same time, the future of the US disposition program also has become clouded. The program has been described as a major nonproliferation effort as it would remove 68 metric tons of plutonium in the two countries and not make it susceptible to potential future diversion.