MOSCOW — Russia’s currency fell on Thursday to its lowest dollar value in nearly two years, jarred by new American sanctions imposed in response to a nerve-agent attack on a former Russian spy in Britain.

For several years, the Russian economy has appeared able mostly to shrug off the effects of a series of Western sanctions tied to its incursions into Ukraine and its election meddling. Buoyed by rising oil prices, the economy even grew modestly this year.

But the ruble’s precipitous decline may be a clear sign that the sanctions have begun to cloud the country’s long-term economic outlook.

As the ruble slumped through the day, officials in Moscow expressed dismay that the new restrictions came so soon after what they saw as a positive summit meeting between President Trump and Vladimir V. Putin, in Helsinki, Finland last month.