Iran's powerful Guardian Council on Sunday rejected a bill on joining the UN convention against terrorist financing seen as crucial to maintaining trade and banking ties with the world.
The conservative-dominated council, which oversees legislation passed by the parliament, said aspects of the bill were against Islamic law and the constitution and sent it back to lawmakers for revision.
"The Guardian Council has in several sessions reviewed the bill... and it has considered it to have flaws and ambiguities," wrote spokesman Abbas Ali Kadkhodaie on Twitter.
The bill, narrowly passed by parliament on October 7, is one of four put forward by the government of President Hassan Rouhani in order to meet demands set by the international Financial Action Task Force (FATF), which monitors countries' efforts to tackle money-laundering and terrorist financing.
Many hawks in Iran say the laws would limit the country's ability to support "resistance groups" such as Lebanon's Hezbollah and Palestinian Hamas by bringing greater transparency to its accounts.
But Rouhani's government argues it is particularly vital after the United States walked out of the 2015 nuclear deal and reimposed sanctions.
The other parties to the deal -- Britain, France, Germany, China and Russia -- have sought to salvage the agreement and maintain trade with Iran, but have demanded that it accede to the FATF.
Iran is alone with North Korea on the FATF, although the Paris-based organisation has suspended counter-measures since June 2017 while Iran works on reforms.
Last month, the FATF gave Iran another extension to February to update its laws.
"Neither I nor the president can guarantee that all problems will go away if we join (the UN convention), but I guarantee that not joining will provide the US with more excuses to increase our problems," said Iran's Foreign Minister Mohammad Javad Zarif during the parliament debate last month.
A previous bill on the mechanics of monitoring and preventing terrorist financing was signed into law in August.
But two others -- on money-laundering and organised crime -- have also been delayed by higher authorities, including the Guardian Council, after being approved by parliament.
The council is made up of six clerics appointed by supreme leader Ayatollah Ali Khamenei and six lawyers appointed by the judiciary.
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