Tuesday's sanctions target Iran's purchases of USA dollars, metals trading, coal, industrial software and the auto sector.
Meanwhile, Iranian Foreign Minister Mohammad Javad Zarif said his country will not oppose third-country mediation between the US and Iran.
During his meeting with North Korean Foreign Minister Ri Yong-ho in Tehran on Wednesday, Rouhani warned him that U.S. was "an unreliable and untrustworthy" negotiating partner that did not honour "any of its commitments".
Washington's unilateral anti-Iran campaign has nothing to do with global law and fair trade, and thus the threat of potential sanctions won't affect Beijing's sovereign right to freely choose its business partners.
Ri was also quoted as describing the sanctions against Iran as "inappropriate and contrary to worldwide standards".
Following its withdrawal from the Iran nuclear deal earlier this year, on Tuesday the USA slapped Iran with "the most biting sanctions ever".
"China's commercial cooperation with Iran is open and transparent, reasonable, fair and lawful, not violating any United Nations Security Council resolutions", it said.
Signed in 2015, the landmark deal was meant to lift sanctions against Iran in return for curbs in its nuclear programme.
On May 8, Trump withdrew unilaterally the US from the 2015 nuclear deal, known as the Joint Comprehensive Plan of Action, and ordered restoration of sanctions against Iran. At current market rates, the imports are worth some $15 billion a year.
China and Germany defended their business ties with Iran on Wednesday in the face of President Donald Trump's warning that any companies trading with the Islamic Republic would be barred from the United States.
But the second tranche of sanctions, which kicks in on November 5 and targets Iran's vital oil sector, could be far more damaging - even if several key customers such as China, India and Turkey have refused to significantly cut their purchases.
On Tuesday, north Korean state media implored the USA to lift its sanctions accusing Washington of leveraging the punitive measures to "raise its negotiating power".
European countries, hoping to persuade Tehran to continue to respect the deal, have promised to try to lessen the blow of sanctions and to urge their firms not to pull out.
The Iranian rial fell to a record low late last month, dropping in value by half since April, leading to sharp price increases and fast-rising inflation.
"I feel like my life is being destroyed", said one construction worker on the streets of the capital.