Iran’s Asian friends as well as Europeans have expressed their keenness to continue trade with Tehran after the United States reinstated its "toughest sanctions ever" on the Islamic Republic on Monday.

China's Foreign Ministry spokeswoman Hua Chunying said the Asian giant's lawful trade cooperation with Iran should be respected and expressed regret about the US reimposing sanctions on Tehran.

"China expresses regret at the US' decision. We also noted that the international community is widely against unilateral sanctions," he told a daily news briefing in Beijing.

"China consistently rejects unilateral sanctions and long arm tactics. We think China and Iran carrying out normal cooperation under the framework of international law is lawful and reasonable, and (this right) should be respected and protected," he added.

The European Union also said it is opposed to the US decision, under which the second batch of sanctions targeting Iran's oil and financial sectors were put into effect.

"The European Union does not approve of it," European Economic Affairs Commissioner Pierre Moscovici told franceinfo radio on Monday, hours after the sanctions were reinstalled.

According to US Treasury Secretary Steven Mnuchin, the sweeping sanctions will see 700 people blacklisted, including people who were granted relief under the 2015 nuclear deal, as well as over 300 new names in Iran's oil, shipping, insurance and banking sectors.

Washington, however, had to go back on its pledge to zero Iran's oil exports, allowing eight top customers of the country to continue their purchases.

A South Korean government official said on Monday that Seoul had been granted an exemption after it called for "maximum flexibility" from Washington.

"Petrochemicals are key to our economy, and we stressed that if we're hit by the Iran sanctions, it would pose grave challenges to our whole economy," said the official who spoke on condition of anonymity.

South Korea was also granted an exemption from Washington to continue financial transactions with the sanctioned Iranian central bank to facilitate oil imports, according to the official.

Other countries exempted from the sanctions are Iran's top oil customers, China and India, as well as Japan and Turkey.

Switzerland said on Monday it is holding talks with the US and Iran about launching a payment channel to help ensure that food and drugs keep flowing to the Islamic Republic.

"Switzerland is committed to safeguarding Swiss economic interests and closely follows the development of the situation. The authorities are in direct contact with the competent authorities of the United States, the EU and Iran," the State Secretariat for Economic Affairs (SECO) said.

"Particularly in the humanitarian field, the federal government is committed to ensuring that food and pharmaceutical products can continue to be supplied from Switzerland," it said.

SECO said it was in touch with US authorities, Iran and unnamed Swiss companies on developing a humanitarian payment channel.

The Europeans are working to set up a special payment system that would bypass the US sanctions and discard the dollar in dealings with Iran.