Iran said it would exceed limits on its enriched-uranium stockpiles before the end of this month, as the U.S. said it would send an additional 1,000 troops to the Mideast in response to “hostile behavior” by Tehran.

Iran’s threat on Monday came days after the U.S. accused Tehran of orchestrating a second set of attacks on oil tankers near a vital global-shipping route. If carried out, Iran would violate the 2015 nuclear deal by breaching the pact’s enriched-uranium cap. That could jeopardize European efforts to save an accord...