Biden to use rest of term putting Ukraine in 'best position': advisor

US President Joe Biden will use the remaining four months of his term "to put Ukraine in the best possible position to prevail", a close advisor said Saturday.

Speaking remotely to a forum in Kyiv, US National Security Advisor Jake Sullivan also said Biden will meet Ukrainian President Volodymyr Zelensky in late September at the UN General Assembly in New York to discuss aid to Ukraine.

"President Zelensky has said that ultimately this war has to end through negotiations, and we need them to be strong in those negotiations," Sullivan said, adding Ukraine would decide when to enter talks with Russia.

Biden will be replaced next January either by his Vice President Kamala Harris, who has indicated she will continue his policies of backing Ukraine, or by former president Donald Trump, who would not say at a debate earlier this week whether he wanted Kyiv to win the war.

The announcement of the upcoming Biden-Zelensky meeting came after Moscow and Kyiv earlier Saturday swapped 103 prisoners of war each in a UAE-brokered deal, and as Russian forces continue to gain ground in their grinding offensive in east Ukraine.

The Russians released in the latest swap were captured during Ukraine's recent cross-border incursion into the Kursk region, Moscow said, while some of the Ukrainians freed had been held prisoner since Moscow seized the Azovstal steel plant in May 2022.

"Another 103 soldiers were returned to Ukraine from Russian captivity," Zelensky said on Telegram.

Russia confirmed it had "handed over" 103 Ukrainian army prisoners, and in return received 103 Russian servicemen captured by Kyiv.

"All Russian servicemen are on the territory of the Republic of Belarus, where they are being provided with the necessary psychological and medical assistance," the Russian defence ministry said.

Despite ongoing hostilities, Russia and Ukraine have managed to swap hundreds of prisoners throughout the two-and-half-year conflict -- often in deals brokered by the United Arab Emirates, Saudi Arabia or Turkey.

The announcement came a day after Zelensky said 49 Ukrainian POWs had been returned from Russia, and three weeks ago both sides swapped 115 prisoners each in a deal also mediated by the UAE.

- Russian advances -

The prisoner swap came as Russia pushed ahead in east Ukraine, where it claims to have captured a string of villages in recent weeks.

The Russian defence ministry said in a daily briefing it had "liberated" the village of Zhelanne Pershe, less than 30 kilometres (19 miles) from the key logistics hub of Pokrovsk.

Pokrovsk lies on the intersection of a key road that supplies Ukrainian troops and towns across the eastern front and has long been a target for Moscow's army.

More than half of the city's 60,000 residents have fled since the invasion began in February 2022, with evacuations ramping up in recent weeks as Moscow's army closes in.

Ukraine had hoped its major cross-border incursion into the Kursk region last month would slow down Russia's advances in the east.

On Friday, Zelensky said Moscow had been slowed down somewhat but conceded the situation on the eastern front was "very difficult".

"Ukraine has taken bold and assertive steps... but that area around Pokrovsk is of unique concern," Sullivan told the Kyiv forum on Saturday.

Russia meanwhile claimed this week to have clawed back a swath of territory in its western Kursk region, as it mounted what appeared to be a counter-offensive.

- Missile spat -

Tensions between Russia and the West over the conflict reached dire levels this week over UK and US discussions about letting Ukraine use longer-range weapons to strike targets inside Russia.

President Vladimir Putin warned the West on Thursday that green-lighting the use of the long-range weapons deep inside Russia would put the NATO military alliance "at war" with Moscow.

"It would mean that NATO countries, the US, European countries, are at war with Russia," Putin told a state television reporter.

British Prime Minister Keir Starmer and US President Biden on Friday delayed a decision on the move.

Sullivan, in his comments by video link to the forum in Kyiv, said "difficult and complicated" logistics -- rather than unwillingness -- was delaying aid to Ukraine.

"It's not a matter of political will," Sullivan said. "But given what Ukraine is up against, we've got to do more, and we've got to do better."

Speaking at the same forum, Ukraine's intelligence chief Kyrylo Budanov said that of all of Russia's allies, North Korea's assistance is the most damaging because of the large quantities of ammunition it provides.