Plans for two US peace envoys to visit Russia and Ukraine this weekend have yet to be finalized because of US security concerns, a source familiar with the situation told Reuters.
Russian state news agency TASS and Axios separately reported that Steve Witkoff and Jared Kushner would visit Moscow and Kyiv in the next two days.
However, the source told Reuters that the US was still seeking commitments from both sides to ensure the safety of the two negotiators before they could travel to Moscow on Saturday and Kyiv on Sunday.
"The plan is to visit Moscow and Kyiv but we hear that Witkoff is nervous," the source said. "We hear the signals that Witkoff is not so sure as (he was) some days ago."
The source was speaking shortly before a Russian attack struck the headquarters of Ukraine's SBU security service in Kyiv on Friday, sending smoke rising over the city in the 10th day of strikes on the capital.
Russia and Ukraine have intensified their air war in recent months, with Moscow raining drones and missiles on Kyiv and Black Sea ports while Ukraine sends hundreds of drones at a time to attack Russian refineries, oil terminals and commercial warehouses.
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Ukrainian President Volodymyr Zelensky warned commercial airlines this week to avoid Russian airspace, saying it was too dangerous. Russian President Vladimir Putin said that amounted to a threat of state terrorism.
The source told Reuters that Washington was seeking commitments that Ukraine would not strike Moscow and Russia would not attack Kyiv during the envoys' trip, and that terms for such an agreement were currently under discussion.
Kremlin spokesman Dmitry Peskov was asked on Friday if a Witkoff-Kushner visit to Moscow would require an agreement by Ukraine not to do anything that would pose a danger during their visit.
He replied that the US would likely demand such a commitment from Kyiv.
Asked if there would be any direct agreements between Russia and Ukraine on the issue, Peskov replied: "No, there won't be."
Huge gaps separate two sides
A vast gulf remains between Russia and Ukraine on how to end the 4-1/2-year-old war, the deadliest in Europe since World War Two.
Peskov said the position that Putin set out in a speech two years ago - that Ukraine should cede the entire territory of four regions claimed by Russia and abandon its plan to join NATO - was "unshakeable."
Ukraine refuses to surrender land that it has successfully defended at huge cost, and rejects such terms as tantamount to surrender.
A source close to the Kremlin, speaking before reports of the possible trip by US President Donald Trump's envoys, said Putin was still intent on capturing more territory.
"There is no chance of reaching an agreement regarding the front line until Putin finishes securing the remaining part of the Donetsk region," the source said, adding that the Kremlin leader was confident - based on reports from his commanders - that this would happen by the end of the year.
Western military analysts see such a possibility as remote, given Moscow's slow rate of progress.
Witkoff and Kushner last visited Moscow in January, when they held four hours of late-night talks with President Vladimir Putin in the Kremlin. They have yet to visit Ukraine.
The last US-mediated talks between Russia and Ukraine took place in February, shortly before the United States and Israel launched a war against Iran.