US President Donald Trump unilaterally withdrew from the 2015 Iran nuclear deal last year and reimposed punishing sanctions as part of a stated campaign of "maximum pressure" against the Islamic republic.

Officially, the punitive measures make exceptions for food, medicine and other humanitarian goods, but most companies are unwilling to do any trade with Iran for fear of repercussions in the world's largest economy.

Trump "administration officials claim they stand with the Iranian people, but the overbroad and burdensome US sanctions regime is harming Iranian's right to health, including access to live-saving medicines", said Sarah Leah Whitson, Middle East director at HRW.

"The comprehensive web of US sanctions has led banks and companies to pull back from humanitarian trade with Iran, leaving Iranians who have rare or complicated diseases unable to get the medicine and treatment they require," she added.

The sanctions include previously suspended nuclear-related embargoes including on Iran's oil exports and financial transactions, with new ones added.

The US Treasury said they were imposed to make Iran's leaders "cease support for terrorism, stop proliferating ballistic missiles, end destructive regional activities, and abandon their nuclear ambitions".

In a 47-page report, HRW documents how the US-built exemptions for humanitarian imports into its sanctions regime have failed to offset the strong reluctance of US and European companies and banks to finance humanitarian goods.

Iranian patients have struggled with a foreign medicine shortage and price hikes for over a year both due to reimposed US trade sanctions as well as a battered economy with a free-falling currency.

Medicine importers get subsidised currency rates from the government, yet foreign drugs and medical equipment cannot always be found in state-owned pharmacies.

Iran produces 96 percent of the drugs it uses but imports more than half the raw materials to make them, according to the Syndicate of Iranian Pharmaceutical Industries.

It also has to import special medicine which patients with rare diseases require.

HRW called on the US to "get serious about addressing the harm resulting from its cruel sanctions regime".

Washington must create "a viable financial channel with reasonable requirements for companies, banks and groups to provide humanitarian goods for people in Iran," it said.