WASHINGTON — Under pressure from Congress to demonstrate that it is not easing up on sanctions on Iran’s oil sector or on its nuclear and missile programs, the Obama administration on Thursday announced an expanded list of companies and individuals that it said it would target to block their trading activities around the world.

Among the newly penalized companies is a Singapore-based firm called Mid Oil Asia, which is accused of helping the National Iranian Tanker Company make payments for services through money transfers that made no mention of the vessels that were aided, or their Iranian ownership. Another Singapore company, Singa Tankers, is accused of helping Iran make “urgent payments.” The location of both companies is notable because Singapore often prides itself on running a carefully regulated shipping and banking system.

Five companies are accused of helping Iran’s nuclear and missile program, including an Iranian firm, the Eyvaz Technic Manufacturing Company, that the United States said had procured some of the most sensitive and hard-to-build components for Iran’s nuclear centrifuges. The centrifuges are the machines that, spinning at supersonic speeds, enrich uranium; over the years the United States has sought to undermine the effort with sanctions, faulty parts and cyberattacks.

Another firm is accused of helping Iran obtain components for its heavy-water reactor facility, which officials fear will ultimately give Iran another pathway to a bomb capability, using plutonium.