John Tefft also stressed that Washington cannot ignore Russia's "fundamental challenge to the international order" in the Ukraine crisis.
But the defunct oil giant's former vice president, Leonid Nevzlin, also said it might take some time to see any of the $50 billion.
Washington puts three banks and a shipbuilder on a list of companies to which U.S. nationals and firms are barred from providing longer-term financing or issuing equity.
Under Secretary of State for Political Affairs Wendy Sherman, the top negotiator for the United States in the P5+1 talks with Iran over its nuclear program, declined to give a hard date for when negotiations would end.
Mail.ru is worried about an ongoing block of its social-media site in Tajikistan.
Russian lawmakers have reportedly drafted amendments that would brand states that impose sanctions on Russia as "aggressor countries."
The secretary-general of the Shanghai Cooperation Organization (SCO) has arrived in Tajikistan ahead of a meeting of the organization.
Senior officials of the former Kosovo Liberation Army face indictment by a special EU court for alleged killings, abductions, and sexual violence.
Fighting between government forces and pro-Russian separatists has again prevented international investigators from going to the MH17 crash site in eastern Ukraine.
Eighty-three tourists from 11 countries have not been allowed to enter the Chukotka Peninsula in Russia's Far East.
The European Union has agreed to impose additional sanctions on eight individuals, including four close business associates of Russian President Vladimir Putin, and four entities over the Ukraine crisis.
The trial of Belarusian human rights activist Andrey Bandarenka has started in Minsk.