He made the remarks during a telephone conversation with French President Emmanuel Macron late on Tuesday when he added that today the Iranian nation feels deeply grieved due to the US crime.
"By assassinating Lieutenant General Soleimani, the Americans committed a big strategic mistake," he reiterated.
Rouhani hoped that all the countries favoring sustainable peace and security in the world would adopt decisive stance against the terrorist attack.
Noting that the US aims from the terror act have not been fulfilled and its hostile policies will surely backfire, he added that after the crime unity and solidarity among the Iranian people have improved and the Iranian and Iraqi nations have closer to each other more than before.
Lieutenant Soleimani was in Baghdad on the invitation of the Iraqi government for a meeting with its prime minister, the chief executive said, noting that the US act is violation of Iraq's sovereignty, insulting the country's people and government as well as a big and unforgivable crime against the Iranian people.
Referring to the Iranian people's demand to respond to Washington, he pointed out that responsibility of the assassination and its consequences lies with the US government.
"Americans should know that their security and interests across the region are at risk and they will not remain immune to repercussions of the big crime," he reiterated.
Regional security is Iran's major strategy, he said, adding that Soleimani's objective as establishing sustainable security and stability in the region and he was champion of fighting terrorism.
Rouhani also hailed Iraqi government's major decision in reaction to US terrorist attack, saying that regional security will be ensured only by the regional states.
Nobody will benefit from instability in the region, as it can be threatening for the entire world, he said.
The Islamic Republic of Iran has never sought war and insecurity in the region but it will not hesitate to defend its rights and sovereignty, Iranian president said.
Pointing to the fifth nuclear step, he said that what has been done in the process of five steps were all in line with the JCPOA, but the opposite party did not fulfill its commitments and this has caused imbalance in the international deal.
Whenever the other parties do their parts under the deal, the steps so far taken will be reversible, President Rouhani said.
President Macron, for his part, said that he understands the bitter and grievous conditions under which he is talking to Rouhani, adding, "All of us are worried about peace and stability in the region and the incidents occurred can help escalate irreparable tensions."
Referring to his country's permanent commitment to establish peace and stability, the French president regretted over the recent incidents and voiced his readiness for easing the tensions.
France will continue its efforts to preserve the JCPOA as a major international agreement, he said, noting that it expects Iran to move on the same path.
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