The US allegations are the latest salvo against Iran from the administration of President Donald Trump, who earlier this year pulled out of a landmark 2015 nuclear deal between major powers and Tehran and reimposed crippling unilateral sanctions.
"(The) US wants to resort to international conventions to make allegations against Iran when it's made a policy of violating them itself," Zarif said in a tweet.
He said that allegations about weapons of mass destruction "by a country that supported Iraq's use of CW (chemical weapons) against Iran, then invaded Iraq to allegedly rid it of them is not just obscene, it's dangerous".
Earlier on Friday the Iranian foreign ministry said Tehran "strongly rejects" the US accusations.
"The United States... has made, as is its habit, baseless accusations against the Islamic republic which we strongly reject," the foreign ministry said in a statement.
"Such incorrect and false accusations are due solely to enmity towards the Iranian nation and are intended to deflect international attention from its own broken commitments and continued support for the Zionist regime's chemical arsenal and for terrorist groups."
The US accused Iran on Thursday of failing to declare a chemical weapons programme to the global watchdog in breach of international agreements.
US envoy Kenneth Ward told the Organisation for the Prohibition of Chemical Weapons in The Hague that Tehran was also seeking deadly nerve agents for offensive purposes.
Iran has accused the US of being the "only member state that has a chemical weapons arsenal and has, until now, not acted on its obligations to destroy it".
Iran is one of the few countries that has been attacked with chemical weapons since the end of World War I in 1918.
Chemical weapons used by Saddam Hussein's Iraq during its 1980-88 war with Iran killed and wounded tens of thousands of Iranian soldiers and civilians.
Iran has expressed strong opposition ever since to the use or possession of chemical weapons by any country.
Zarif, who is in Italy for a conference, on Friday also expressed his frustration with the European Union's delay to come up with means of allowing EU trade with Tehran despite US sanctions .
"(The) EU needs to make a decision: does it want the US to dictate its relations with others...?" he asked in a tweet.
The nuclear deal "was in the interest of the security of Europe (which) now needs to invest in that security." he added.
Although the European Union has vowed to create a "special purpose vehicle" (SPV) to protect companies buying oil, analysts see little chance that firms will risk US penalties by using it.
The US allegations are the latest salvo against Iran from the administration of President Donald Trump.