Rohani stopped short of saying who had used the arms – Tehran has previously accused Syrian rebels of being behind what it called suspected chemical attacks.
He also did not mention the international furor around Syrian opposition reports that forces loyal to the Damascus government killed as many as 1,000 civilians with poison gas in suburbs of Damascus on Wednesday.
"Many of the innocent people of Syria have been injured and martyred by chemical agents and this is unfortunate," recently elected Rohani was quoted as saying by the ISNA news agency.
"We completely and strongly condemn the use of chemical weapons," he said, according to the agency.
"The Islamic Republic gives notice to the international community to use all its might to prevent the use of these weapons anywhere in the world, especially in Syria," he added, according to the Mehr news agency.
Syria's government denies using such weapons and Iran's foreign minister earlier this week said groups fighting Syrian president Bashar Assad in a two-year-old rebellion must have been behind what he then said was just a suspected attack.
Russia, another major ally in the Syrian government, has also blamed opposition forces.
Syria's uprising against four decades of Assad family rule has turned into a civil war that has killed more than 100,000 people.
Foreign powers have said chemical weapons could change the calculus in terms of intervention and are urging the Syrian government to allow a UN team of experts to examine the site of Wednesday's reported attacks.
The United States on Friday was repositioning naval forces in the Mediterranean to give President Barack Obama the option for an armed strike on Syria, although officials cautioned that Obama had made no decision on military action.
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