US President Donald Trump's envoys Steve Witkoff and Jared Kushner are likely to visit Kyiv and Moscow this weekend as part of Washington's efforts to advance peace talks between Russia and Ukraine, with their first-ever visit to the Ukrainian capital expected on Sunday, according to reports.

Witkoff and Kushner, Trump's son-in-law, have been involved in US-led shuttle diplomacy between Russia and Ukraine and have travelled to Moscow several times.

However, according to Reuters, citing a source, plans for the two US peace envoys to visit Russia and Ukraine have yet to be finalised due to security concerns.

Russian state news agency TASS and US news outlet Axios separately reported that Witkoff and Kushner would travel to Moscow and Kyiv over the next two days.

According to Reuters, the US was still seeking security commitments from both Russia and Ukraine before allowing the envoys to travel to Moscow on Saturday and Kyiv on Sunday.

"The plan is to visit Moscow and Kyiv but we hear that Witkoff is nervous," Reuters quoted the source a saying. "We hear the signals that Witkoff is not so sure as (he was) some days ago."

The source spoke shortly before a Russian attack struck the headquarters of Ukraine's SBU security service in Kyiv on Friday, sending smoke over the city as the capital faced its 10th consecutive day of strikes.

Russia and Ukraine have intensified their air war in recent months. Moscow has continued to launch drones and missiles at Kyiv and Black Sea ports, while Ukraine has carried out waves of drone attacks targeting Russian refineries, oil terminals and commercial warehouses.

Meanwhile, Ukrainian President Volodymyr Zelenskyy said on Friday that Kyiv has proposed observing a ceasefire with Russia while US envoys  visit Moscow and Kyiv.

"There will be no airstrikes on our part, and Russia must reciprocally ensure a ceasefire - without its own airstrikes - for the duration needed to conduct these talks," AFP quoted Zelenskyy as saying in his evening address.

Ukrainian President Volodymyr Zelenskyy warned commercial airlines this week against flying through Russian airspace, saying it was too dangerous. Russian President Vladimir Putin, however, said the warning amounted to a threat of state terrorism.

According to Reuters, citing a source, Washington was seeking assurances that Ukraine would not attack Moscow and Russia would not strike Kyiv while the US envoys were in the region. Discussions on the terms of such an understanding were underway, the source said.

Kremlin spokesman Dmitry Peskov was asked on Friday whether a potential visit by Witkoff and Kushner to Moscow would require Ukraine to agree not to take any action that could endanger them during their stay.

Peskov said the US would likely seek such a commitment from Kyiv.

Asked whether Russia and Ukraine would reach any direct agreement on the matter, Peskov said: "No, there won't be."

Huge gaps between two sides

A wide gulf remains between Russia and Ukraine over how to end the 4-1/2-year-old war, the deadliest conflict in Europe since World War Two.

Peskov said the position outlined by Putin in a speech two years ago — that Ukraine should cede the entirety of four regions claimed by Russia and abandon its ambition to join Nato — remained "unshakeable".