WASHINGTON—An Iranian oil executive removed from U.S. financial blacklists last month because he had resigned from Iran’s national oil company now holds senior positions with two Iranian energy firms linked to the Iranian government, according to company officials and regulatory filings.

The executive’s role at the two companies appears to violate U.S. sanctions, analysts said, and is expected to fuel concern among U.S. lawmakers critical of the administration’s Iran policy. Both companies contract with Iran’s state energy firms, including the blacklisted National Iranian Oil Company, and one of the firms is substantially owned by a sanctioned state-owned Iranian bank.

Former NIOC Chief Executive Ahmad Ghalebani was taken off the U.S. Iran sanctions list on June 11 because he no longer held that position in the state firm, “a verified change in behavior” according to the Treasury Department. Mr. Ghalebani stepped down from that post in 2013, the same year he was originally blacklisted by the U.S.

The sanctions removal, made at Mr. Ghalebani’s request, lifted financial blocks from any assets he may have had within the U.S. and affords him unfettered access to the U.S. and other foreign markets. Critics said the removals weakened the U.S.’s negotiating position, among other objections.

Mr. Ghalebani continues to serve as an adviser to the chief executive of Iranian oil-services firm Global Petro Tech Kish, according to a representative of the company, and was chosen as chairman of a shareholders meeting in late May of petrochemical firm AzarAb Industries, according to a filing with the Iranian stock exchange.

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The Treasury Department, which oversees U.S. sanctions, didn’t comment on The Wall Street Journal’s findings about Mr. Ghalebani. The State Department didn’t respond to a request for comment.

Mr. Ghalebani, through his companies, didn’t respond to requests for comment.

A spokesman for Iran’s Foreign Ministry, Saeed Khatibzadeh, said last month that the Iranian government didn’t view the delistings as “signals of goodwill” and urged the Biden administration to remove all sanctions.

U.S. sanctions law permits the Treasury Department to impose sanctions on individuals who do business with the blacklisted government of Iran, including NIOC, or those who work in Iran’s energy sector. The U.S. has long used sanctions against Iran to coerce Tehran into abandoning its nuclear program and curbing its involvement in regional conflicts and support for U.S.-designated terror groups.

The Biden administration said that even though the delistings of Mr. Ghalebani and two other former officials in Iran’s oil sector were meant to send a message that the U.S. is serious about sanctions relief for changes in Iran’s behavior, they weren’t connected to ongoing nuclear negotiations. Rather, U.S. officials said, it was part of a normal, law-based process after the men filed petitions for removal.

The Treasury’s sanctions unit “determined, through a rigorous, evidence-based, administrative reconsideration process, that the individuals are no longer in these positions within government of Iran entities,” a spokeswoman said. The department “continues to monitor for any changes in circumstances” that led to their delisting, she said.

Jason Brodsky, the former policy director for United Against a Nuclear Iran and now a senior Middle East analyst at London-based Iran International TV, said Mr. Ghalebani’s ongoing activities appear to be violating sanctions, given the expansive reach of the powers authorizing punitive action against Iran.

Besides working with a company that signed a major drilling contract with NIOC last year, Mr. Brodsky said, Mr. Ghalebani is also affiliated with AzarAb Industries, which “means he is still connected to a regime-affiliated company that was sanctioned by the EU for providing manufacturing support to Iran’s nuclear program.”

Critics of the Biden administration’s Iran policy say the June delistings of Mr. Ghalebani and the other two officials foreshadow a new nuclear agreement with Iran that lifts the broader sanctions campaign without securing commitments they believe are critical to national security.

Mr. Ghalebani served as board chairman and chief executive at AzarAb Industries and Global Petro Tech Kish, respectively, until a few weeks ago, according to corporate records and a public-relations officer at one of the firms.

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Mr. Ghalebani’s past and present roles suggest he remains an important figure among Iran’s political and economic elite, Iran analysts say. The lifting of the sanctions came amid stalled nuclear negotiations and signaled the Biden administration’s willingness to further ease economic pressure on Iran.

“It’s unlikely that Iranian oil executives and sanction-busters have suddenly ditched their skill set,” said Behnam Ben Taleblu, a senior fellow specializing in Iran and arms control at the Foundation for Defense of Democracies, a Washington-based think tank that has advocated more-stringent sanctions.

Jason Bartlett, a sanctions analyst at the Washington-based Center for a New American Security, said that because the men were blacklisted for their official status, “we cannot legally continue sanctioning those individuals, because they have no more affiliation that’s sanctionable.”

“It can show that we’re following the rules of sanctions and not using them in an indiscriminate, punishing, unilateral way,” Mr. Bartlett said.

Neither AzarAb nor Global Petro Tech Kish are on the U.S. sanctions list. About 30% of AzarAb is owned by Bank Keshavarzi, a state-owned agricultural bank sanctioned by the U.S. government, according to AzarAb’s Iranian regulatory filings. The company is a major contractor to NIOC and other state-owned energy firms, including in building and servicing petrochemical plants that refine the country’s oil and natural gas. Global Petro Tech Kish contracts with sanctioned NIOC subsidiaries on some of Iran’s largest and most important oil fields, according to the company’s list of current projects, contracts listed on the Tehran Stock Exchange and Iranian news reports.

NIOC is currently under U.S. sanctions for its alleged role funding the Islamic Republic of Iran’s weapons programs banned by the U.S. and other Western nations. Iranian military units that the U.S. and other Western countries have labeled terrorist organizations are also under sanctions. The Treasury Department says it is authorized to impose sanctions against any person or entity found “to have provided material support for, or goods or services in support of the National Iranian Oil Company.”

—Aresu Eqbali in Tehran contributed to this article.

Write to Ian Talley at ian.talley@wsj.com