The Obama administration recently sent private messages to foreign governments and U.S. bankers reminding them that sanctions against Iran are still in effect and therefore they should not yet invest in Iran's oil industry or other sectors, reported Reuters.

The U.S. State Department cabled the message to embassies around the world, saying that sanctions remain until Iran fully complies with the nuclear agreement reached with six world powers in July.

The message said that before the sanctions on Iran can be lifted, the International Atomic Energy Agency must verify that Iran is complying with the terms of the deal.

U.S. officials have also held meetings with U.S. bankers and oil industry executives in New York City and Washington to remind them that the sanctions are still in effect, according to Reuters.

Under the nuclear agreement, Iran agreed to temporarily halt aspects of its nuclear program in exchange for the lifting of U.S. and United Nations sanctions, which have devastated Iran's oil and gas sector.

It is not exactly clear when sanctions will be lifted to allow for international financial transactions with Iran, but American experts estimate that it will take Iran six to nine months to bring its nuclear program into alignment with the terms of the agreement, according to The New York Times.

Tehran still needs to reduce its 12-ton stockpile of low-enriched uranium to no more than 660 pounds, which is about a 98 percent reduction. Among other uncompleted tasks, it also has to disassemble and store more than 13,000 centrifuges, as it is only allowed to have 5,060 spinning.

In the meantime, if a financial institution violates the sanctions by doing business with the state-owned National Iranian Oil Company or its subsidiaries, U.S. law permits the Obama administration to freeze assets and cut the institution off from the U.S. dollar banking system, Reuters reported.

Hundreds of companies from a number of countries – including Germany, Spain, France, Britain, Italy, India and Japan – have traveled to Iran over the past few months to begin establishing trade deals.

These include some of the world's top oil companies, like Royal Dutch Shell, France's Total and Italy's Eni, who are holding discussions on developing Iran's massive oil and gas reserves, which account for about 9.3 percent of the world's reserves. Part of Shell's deal will involve it repaying its $2 billion debt to the National Iranian Oil Company, according to The International Business Times.