Iran replaced the head of its state-oil company on Sunday amid tension within the government over how to handle the return of foreign investment after Western sanctions lifted in January.

Ali Kardor, who as National Iranian Oil Co.’s investment chief played a key role in bringing foreign companies back to the country, will succeed Rokneddin Javadi,managing director since 2013, NIOC said on its website.

Mr. Javadi, who resigned, has been appointed deputy minister in charge of supervising hydrocarbon resources, the company said.

The changes by Oil Minister Bijan Zanganeh come after government officials expressed disagreement over how to handle foreign investment and what kind of terms should be offered to foreign companies. Those differences of opinion have contributed to delays in tendering new oil-development contracts, two people familiar with the process said.

In remarks carried by the oil ministry’s news agency Shana on Sunday, Mr. Zanganeh acknowledged that contracts for new oil projects had faced criticism and said he was optimistic the first ones could be signed within three months. Conservative lawmakers and members of a paramilitary force called the Basij have frequently written on their websites that deals could be too generous to foreigners.

Mr. Javadi has also recently been criticized by conservatives in Parliament over an export deal with an Emirati company a decade ago, conservative news agencies Fars and Mehr reported. At the time, Mr. Javadi was an NIOC executive. The conservatives, the news agencies reported, claim Mr. Javadi negotiated gas prices that were too low. A spokeswoman for the oil ministry and Mr. Javadi couldn’t be reached for comment.

While it hasn’t signed new deals to develop its fields, Iran has resumed sales of its oil to companies in the European Union after a three-year ban. Royal Dutch Shell PLC recently loaded one Iranian crude oil shipment as a spot cargo, NIOC’s marketing chief, Mohsen Ghamsari, said in remarks posted on the company’s website. Shell last week declined to comment on making a shipment to Iran, which would be a first for Shell since sanctions lifted. Mr. Ghamsari said Iran doesn’t have a long-term deal with Shell.

Overall, Iran is selling 400,000 barrels a day to the EU—two-thirds of its presanctions market share—and is planning to boost that level to 700,000 barrels a day, Mr. Ghamsari said in an interview in May.