Ukrainian President Volodymyr Zelenskiy arrived in neighbouring Poland on Wednesday, a Polish presidential aide said, as he begins an official visit to a close ally that has galvanised Western military and political support for Kyiv.

The visit, which was announced this week by Poland but has not been officially confirmed by Kyiv, comes with Ukraine planning to conduct a counter-offensive in the coming weeks or months to recapture occupied land in its east and south.

"I can say that President Zelenskiy has crossed the Polish border," Marcin Przydacz told private broadcaster TVN24 on Wednesday.

In televised comments on Tuesday ahead of the visit, Przydacz had said the visit "should be taken as a sign of trust and of thanking Poland and Poles".

Poland has taken in more than a million Ukrainian refugees over the past 13 months of war. The NATO member has also played an important role in persuading other Western powers to supply battle tanks and other weaponry to Ukraine.

Military deliveries have been vital for Ukraine to fend off and fight back Russian forces that poured over the Ukrainian border in February 2022. Swathes of Ukraine remain occupied in the south and the east.

Ukrainian Defence Minister Oleksii Reznikov said on Tuesday that Kyiv was grateful to Poland for clearing the way for deliveries of MiG fighter jets. Przydacz said earlier this week the first MiGs had already been delivered to Ukraine.

"MiGs from Poland will significantly strengthen our defence, allow us to make our skies safer, save the lives of our citizens and also reduce the destruction caused by Russian attacks," Reznikov wrote on Telegram.

Zelenskiy will meet President Andrzej Duda and Prime Minister Mateusz Morawiecki on the trip, and speak to Ukrainian refugees and Polish members of the public, Przydacz said.

"It will not be a surprise to anyone that the Ukrainian side will ask Poland and other foreign partners for more support... but we must be aware that we as Poland have already really done a lot," he said.

Poland has said talks will focus on developments on Ukrainian front lines, international support and economic cooperation, he said.

Ukraine's presidential office has not said anything in advance about the trip or what it will focus on.

The Polish public overwhelming supports Ukrainians in their war with Russia. An Ipsos poll said 82% of Poles think NATO and European Union countries should back Ukraine until it wins.

Nonetheless, Zelenskiy's visit takes place amid mounting anger in rural Poland over the impact of imports of Ukrainian grain, which have pushed down prices in several states in the EU's eastern wing.

"During talks with President Zelenskiy, we will certainly discuss Ukrainian grain and various agricultural products, because we want any trade with Ukraine not to destabilise our market," Morawiecki said.

Tariffs on Ukrainian agricultural imports may need to be reintroduced if an influx of products pushing down prices in European Union markets cannot be halted by other means, the premiers of five eastern states said in a letter published on Friday to European Commission President Ursula von der Leyen. (Reporting by Pavel Polityuk in Kyiv and Alan Charlish and Pawel Florkiewicz in Warsaw; writing by Tom Balmforth; editing by Mark Heinrich and Lincoln Feast.)