US President Donald Trump said on Sunday he is ready to move to a second phase of sanctioning Russia, the closest he has come to suggesting he is on the verge of ramping up sanctions against Moscow or its oil buyers over the war in Ukraine.

Trump has repeatedly threatened Moscow with further sanctions but withheld them as he pursued peace talks.

The latest comments suggest an increasingly aggressive posture, but Trump stopped short of saying he was committed to such a decision or what a second phase might entail.

Asked by a reporter at the White House if he is ready to move to "the second phase" of sanctions against Russia, Trump responded, "Yeah, I am." He did not elaborate.

Trump has been frustrated by his inability to bring a halt to the fighting after he initially predicted he would be able to end the war in Ukraine swiftly when he took office in January.

The White House did not immediately respond to an email on Sunday seeking comment about what steps Trump was contemplating.

The exchange was a follow-up to Trump's comments on Wednesday defending the actions he had taken already on Russia, including imposing punitive tariffs on India's US-bound exports last month.

India is a major buyer of Russia's energy exports, while Western buyers have cut back in response to the war.

Treasury Secretary Scott Bessent said on Sunday that the US and the European Union could heap "secondary tariffs on the countries that buy Russian oil," pushing the Russian economy to the brink of collapse and bringing Russian President Vladimir Putin to the negotiating table.

This comes as Russia fired its biggest-ever aerial barrage at Ukraine early on Sunday, killing four people and setting government offices in Kyiv ablaze, an attack President Volodymyr Zelensky warned would prolong the war.

Flames could be seen rising from the roof of the sprawling government complex that houses Ukraine's cabinet of ministers in the heart of the city -- the first time it has been hit during the three-and-a-half-year conflict.

Drone strikes also damaged several high-rise buildings in the Ukrainian capital, according to emergency services.

Russia denies targeting civilians in Ukraine.

It said it struck a plant and a logistics hub in Kyiv, with the Russian defence ministry saying "no strikes were carried out on other targets within the boundaries of Kyiv".

"It is important that there is a broad response from partners to this attack today," said Zelensky in his evening address, adding that Putin was "testing the world".

"We are counting on a strong response from America. That is what is needed." (Agencies)