They were the first confirmed executions this year, after Justice Minister Hassan al-Shammari insisted last week that Baghdad would continue to implement the death penalty in the face of widespread calls for it to issue a moratorium.
Iraq executed at least 129 people last year, according to the justice ministry.
"On Thursday (March 14), we executed eight, and then on Sunday (March 17), we executed 10," Deputy Justice Minister Busho Ibrahim told AFP.
He said that all 18 were convicted of terror-related offences, and that all were Iraqi men.
He declined to give a breakdown of where they were from, but said that some had been tried in northern Nineveh province and some in Baghdad, with others in unspecified provinces.
Al-Qaeda's Iraqi affiliate later said that nationwide attacks on March 19 that killed 56 people were "revenge for those whom you (the government) executed."
Iraq's executions have sparked calls for a moratorium from the United Nations, as well as Britain, the European Union and rights groups Amnesty International and Human Rights Watch.
EU foreign policy chief Catherine Ashton said in a statement today that she "deeply" regretted that Iraq had restarted executions with the government having pledged to review cases of those arrested after months of protests in Sunni areas.
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