US President Donald Trump has ordered an additional 25 percent tariff on Indian goods over New Delhi's continued purchase of Russian oil, a key revenue source for Moscow's war in Ukraine.

The tariff, set to take effect in three weeks, comes on top of a separate 25 percent duty entering into force on Thursday, according to the text of the executive order released by the White House.

The order also threatens potential penalties on other countries deemed to be "directly or indirectly importing Russian Federation oil."

"Should a foreign country retaliate against the United States in response to this action, I may modify this order to ensure the efficacy of the actions herein ordered," the statement said.

The new, Russia-related tariffs on New Delhi will take effect in 21 days, while the already-threatened 25 percent tariff is set to take effect on Thursday.

Exemptions remain for items targeted by separate sector-specific duties such as steel and aluminum, and categories that could be hit like pharmaceuticals.

Trump has been ramping up pressure on India after signaling fresh sanctions on Moscow if it did not make progress by Friday towards a peace deal with Kyiv, as Russia's devastating invasion of its pro-western neighbor drags on.

India's national security adviser was in Moscow on Wednesday, media in New Delhi reported, coinciding with a visit by US envoy Steve Witkoff.

India's Foreign Ministry earlier said US pressure to stop it buying Russian oil was "unjustified and unreasonable" and that it would protect its interests.