US President Donald Trump has ordered steeper tariffs on Indian goods over New Delhi's continued purchase of Russian oil.

The additional 25-percent tariff on Indian goods, coming into place in three weeks, stacks atop a separate 25-percent duty entering into force on Thursday, taking the level to 50 percent for many products.

Trump's order also threatens penalties on other countries who "directly or indirectly" import Russian oil, a key revenue source for Moscow's war in Ukraine.

Exemptions remain however for goods targeted under sector-specific duties such as steel and aluminium, and categories that could be hit later, like pharmaceuticals and semiconductors.

Smartphones are in this list of exempted products for now, shielding Apple from a major hit as the US tech titan shifts production from China to India.

India's foreign ministry condemned Trump's announcement, calling the move "unfair, unjustified and unreasonable."

The ministry previously said India began importing oil from Russia as traditional supplies were diverted to Europe over the war – noting that Washington had "actively encouraged" such imports to strengthen "global energy market stability."

But Trump recently raised pressure on India over the oil purchases, threatening new tariffs as part of a campaign to force Moscow into ending its conflict with Ukraine.

The 25-percent additional tariff is lower than a 100-percent level Trump floated last month when he told Russia to end the war in Ukraine within 50 days or face massive new economic sanctions.

The Republican said at the time that these would be "secondary tariffs" targeting Russia's remaining trade partners, seeking to impede Moscow's ability to survive already sweeping Western sanctions.

"This marks a low point in US-India relations," said Farwa Aamer, the Asia Society Policy Institute's director of South Asia Initiatives.

She expects domestic pressure for India to accede to US demands, but said "this will be a tough road to navigate." (AFP)