Speaking to reporters, Trump also said that two nuclear submarines he deployed following an online row with former Russian president Dmitry Medvedev were now "in the region."

Trump has not said whether he meant nuclear-powered or nuclear-armed submarines. He also did not elaborate on the exact deployment locations, which are kept secret by the US military.

The nuclear saber-rattling came against the backdrop of a deadline set by Trump at the end of next week for Russia to take steps towards ending the Ukraine war or face unspecified new sanctions.

The Republican leader said Witkoff would visit "I think next week, Wednesday or Thursday."

Russian President Vladimir Putin has already met Witkoff multiple times in Moscow, before Trump's efforts to mend ties with the Kremlin came to a grinding halt.

When reporters asked what Witkoff's message would be to Moscow, and if there was anything Russia could do to avoid the sanctions, Trump replied: "Yeah, get a deal where people stop getting killed."

- 'Secondary tariffs' -

Trump has previously threatened that new measures could mean "secondary tariffs" targeting Russia's remaining trade partners, such as China and India. This would further stifle Russia, but would risk significant international disruption.

Despite the pressure from Washington, Russia has continued its onslaught against its pro-Western neighbor.

Putin, who has consistently rejected calls for a ceasefire, said Friday that he wants peace but that his demands for ending his nearly three-and-a-half-year invasion were "unchanged."

"We need a lasting and stable peace on solid foundations that would satisfy both Russia and Ukraine, and would ensure the security of both countries," Putin told reporters.

But he added that "the conditions (from the Russian side) certainly remain the same."

Russia has frequently called on Ukraine to effectively cede control of four regions Moscow claims to have annexed, a demand Kyiv has called unacceptable.

Putin also wants Ukraine to drop its ambitions to join NATO.

Ukraine launched a drone attack Sunday which sparked a fire at an oil depot in Sochi, the host city of the 2014 Winter Olympics.

Kyiv has said it will intensify its air strikes against Russia in response to an increase in Russian attacks on its territory in recent weeks, which have killed dozens of civilians.

Russia's Ministry of Defence said on Monday, its air defences intercepted 61 Ukrainian drones overnight.

One person was killed by Russian shelling in the southern Kherson region, Ukraine's military administration said in Telegram post early Monday.

Ukrainian President Volodymyr Zelensky also said Sunday that the two sides were preparing a prisoner exchange that would see 1,200 Ukrainian troops return home, following talks with Russia in Istanbul in July.

Trump began his second term with his own rosy predictions that the war in Ukraine -- raging since Russia invaded its neighbor in February 2022 -- would soon end.

In recent weeks, Trump has increasingly voiced frustration with Putin over Moscow's unrelenting offensive.