Secretary of State Rex Tillerson said Wednesday that US sanctions against Iran targeted the country's "malign behaviours" and were aimed at helping the Iranian people take control of their government.

Tillerson told a New Delhi press conference after talks with India's Foreign Minister Sushma Swaraj that the United States would not block "legitimate" business activities with Iran by India or any other ally.

President Donald Trump's administration has taken a tougher line with Iran, threatening to tear up an international accord on its nuclear programme. 

Tillerson said US actions also sought to limit Iran's other "destabilising activities" in the Middle East, including its ballistic missile development, "their export of arms to terrorist organisations" and involvement in conflicts in Syria and Yemen.

"Our fight is not with the Iranian people. Our disagreements are with the revolutionary regime," Tillerson said.

"We are taking actions to impose sanctions on the regime and in particular the Iranian Revolutionary Guard. It is our objective to deny financing capacity and to disrupt the activities related to these malign behaviours."

Tillerson made it clear that the United States sought to give "support for modern voices inside of Iran" in a bid to bring about an eventual change of government.

"We know there are strong feelings and values inside of Iran that we want to promote in terms of one day the Iranian people being able to retake control of their government.

"They live under this repressive revolutionary regime and we do not want to harm the Iranian people. Our fight is not with the Iranian people," he said.

Tillerson was questioned about India's investment of hundreds of millions of dollars in the Iranian port of Chabahar. The deal was sealed in 2016 when Indian Prime Minister Narendra Modi visited Tehran.

The construction of Chabahar expands a trade route for landlocked countries in central Asia that bypasses Pakistan, India's arch-rival.

Tillerson said he saw "no contradiction" in the US call for sanctions and India's deal in Chabahar, and Washington did not aim "to interfere with legitimate business activities that are going on with other businesses whether they be from Europe (or) India".