WASHINGTON – Iran will get a first installment around February 1 of $550 million of $4.2 billion being unblocked under a six-month landmark nuclear deal with the West, a US official said Sunday.

"The installment schedule starts on Feb. 1 and the payments are evenly distributed" across 180 days, a senior US administration official told AFP.

"That means the installment payments are every 34 days, except the last payment happens on day 180, which is 33 days after the 5th installment payment," the official said in an email, asking not to be named.

However, as February 1 is a Saturday, the first payment might not be made until February 3.

The money is being unblocked under a landmark deal, set to take effect starting January 20, that curbs parts of Iran's disputed nuclear program in exchange for sanctions relief.

Tehran agreed in November to roll back parts of its nuclear work and halt further advances in exchange for the release of $4.2 billion in frozen assets and limited relief from sanctions that have choked its hard-hit economy.

There will be a second installment on March 7 of $550 million, followed by equal installments of $550 million monthly with the last due on July 20, although that again is a Saturday and therefore could take place on July 21.

In addition, a payment of $450 million is planned for March 1 in return for Iran diluting half its stock of 20 percent enriched uranium to no more than 5 percent.

A second similar payment of $450 million will be due on April 15 if Tehran completes the dilution of all its stock of 20 percent enriched uranium.

Under this schedule, Iran will receive two injections of cash in both March and April if, as agreed, it waters down its 20 percent enriched uranium, ensuring it will no longer be weapons-grade material.