US President Donald Trump has praised Prime Minister Narendra Modi, calling him a "good man" for "cutting down" the purchase of Russian oil.

"Modi's a very good man," Trump told reporters on Air Force One, while on his way back to the White House from West Palm Beach in Florida on Sunday (local time).

"He's a good guy," he said while responding to a question.

Trump claimed New Delhi adjusted its policy after understanding Washington's displeasure over continued Russian energy purchases. "He knew I was not happy and it was important to make me happy," Trump said, referring to PM Modi.

"They (Indians) wanted to make me happy, basically. (PM) Modi's a very good man. Yes, he's a good guy. He knew I was not happy, and it was important to make me happy. They do trade, and we can raise tariffs on that very quickly," Trump said in response to a question that was initially being responded to by Senator Lindsey Graham, who was travelling with him on Air Force One.

Graham asserted that the US tariff pressure has already pushed India to significantly reduce its purchases of Russian oil, arguing that the move is weakening Moscow's ability to sustain its war effort in Ukraine and demonstrating the effectiveness of tariffs as a foreign policy tool.

Speaking to reporters, Graham credited Trump's 25 per cent tariff imposed on India for buying Russian crude, saying the measure had produced visible results.

"I really do believe what he did with India is the chief reason India is now buying substantially less Russian oil," he said.

The President warned that tariff pressure could be increased quickly if countries reverse course. "We can raise tariffs on them very quickly," Trump said, adding that trade leverage remains firmly in US hands.

Graham said his legislation would give the President sweeping authority to impose tariffs on countries continuing to buy discounted Russian energy.

"If you're buying cheap Russian oil keeping Putin's war machine going, we're trying to give the President the ability to make that a hard choice," Graham said.

Graham said the bill, backed by 85 co-sponsors, would allow tariffs ranging from zero to 500 per cent, entirely at the President's discretion. He said the approach has already shown results.

"I was at the Indian ambassador's house about a month ago, and all he wanted to talk about is how they're buying less Russian oil," Graham claimed.

"This stuff works," Graham added.