Ukraine President Volodymyr Zelenskyy has made a powerful address to the UK parliament to appeal for fighter jets to aid his country in repelling invading Russian forces.

Before Mr Zelenskyy landed in Britain, the UK government announced plans to expand a programme training Ukraine's military to ensure its pilots could fly sophisticated NATO-standard fighter jets.

The announcement did not go so far as to promise to provide Ukraine with jets for the newly trained pilots to fly, but it did signal a shift in support that could invite other countries to send planes.

The Ukrainian President, however, appealed directly to his hosts for the hardware.

"I appeal to you and the world, with simple and yet the most important words — combat aircraft for Ukraine," he said, speaking to representatives from across Britain's political spectrum in the vast expanse of Westminster Hall in London.

He proceeded to present an air force helmet with the message "we have freedom, give us wings to protect it" to the speaker of the House of Commons, the lower house of parliament.

The pilot it belonged to is "one of the most successful aces, and he's one of our kings", Mr Zelenskyy said.

Mr Zelenskyy referenced his last visit to the country, in 2020 prior to Russia's large-scale invasion.

Two years ago, he said, he had left the British parliament thanking politicians for "delicious English tea".

UK Prime Minister Rishi Sunak was adamant that no decision had been made to supply Ukraine with combat aircraft, but said Defense Secretary Ben Wallace was "actively considering" the possibility.

Russia's embassy in Britain cautioned against sending fighter jets to Ukraine in a statement.

"Bloodshed, next round of escalation and resulting military and political consequences for the European continent and the entire world," the statement read.

The UK is currently leading an international effort, which includes Australian involvement, to provide Ukrainian recruits with basic training to prepare them for the frontline against Russia.

Mr Zelenskyy also met with King Charles in Buckingham Palace, who told the Ukrainian President he had been "quite worried" about him.

"We've all been worried about you and thinking about your country for so long, I can't tell you," the King said.

London was Mr Zelenskyy's first stop on only his second trip abroad since Russia invaded Ukraine almost one year ago, after a visit to Washington in December.

The UK is one of the biggest military backers of Ukraine and has sent the country more than 2 billion pounds ($3.48 billion) in weapons and equipment.

Coinciding with the visit, the UK government also announced a new round of sanctions against six entities that Britain said supplied equipment to the Russian military, including drone manufacturer CST.

He travels to Paris later in the day and is expected in Brussels on Thursday, where EU leaders are scheduled to meet for a summit.

ABC with wires