Ukraine has told the US that accepting curbs on the size of its armed forces or on its military's overall readiness would be a red line, a senior Ukrainian official said, as Donald Trump strives to negotiate an end to the war with Russia.
Russian President Vladimir Putin said he wants the size of Ukraine's army to be limited.
He has also said Ukraine must drop its ambitions to join NATO and that Russia must control the entirety of four Ukrainian regions that it claims as its own.
"This is a principled position of Ukraine - no one, and certainly not the aggressor country Russia, will dictate to Ukraine what kind of armed forces Ukraine should have," the senior official, Pavlo Palisa, told Reuters in an interview.
Mr Palisa is a deputy head of President Volodymyr Zelensky's office and was part of the Ukrainian delegation that met US officials for talks in Saudi Arabia last month.
A well-prepared military would be Ukraine's best security guarantee against new Russian attacks if and when a ceasefire or peace deal is reached, he said.
"I can guess what the Russian Federation is guided by - maybe they want to prepare, to make it easier for themselves in the future, but no. Our task is to learn the lessons (of the past) well," Mr Palisa added.
During a first meeting with US officials in Saudi Arabia, Ukraine agreed to an unconditional 30-day ceasefire proposed by the US after which Washington resumed military aid and intelligence sharing with Ukraine following a brief pause.
But Russia said crucial conditions needed to be worked out before a ceasefire could be reached.
The sides then separately agreed to halt attacks on each other's energy facilities, but have since accused each other of flouting the agreements.
Ukraine said it may hold a new round of talks with US officials next week.
Ukrainian officials said they have shared evidence of Russian attacks on energy infrastructure with the United States, with Mr Palisa insisting Ukraine is honouring the agreement.
Mr Trump's administration has pushed for a quick end to the full-scale war launched by Russia in February 2022, but a lasting peace deal appears far from imminent.
Fighting continues and Ukraine's top general has said a new Russian offensive push is already under way in the northeast of the country, where Russia said it had captured another village in the Ukrainian region of Sumy.
The Russian claims came after Ukrainian officials said 12 people had been wounded in a drone attack on the central industrial city of Nikopol.
In a statement on social media, Russia's defence ministry said its troops had "liberated" the village of Zhuravka, just over the Ukrainian border.
The claimed Russian advances in Sumy point to the broader difficulties of the Ukrainian army that steadily lost ground in Russia's Kursk region after launching their surprise incursion last August.
Ukraine's most senior military official Oleksandr Syrsky warned earlier this week that Russian forces had launched an offensive in Sumy - where it is advancing since pushing Ukrainian troops out of most of the western Kursk region - and the neighbouring region of Kharkiv.
The governor of the central Dnipropetrovsk region said Russia had attacked Nikopol with a drone and that a 16-year-old teenager was in a serious condition.
That attack came after around one dozen people wounded in overnight Russian attacks on Kyiv and the southern Black Sea region of Mykolaiv.
AFP journalists in the capital heard air raid sirens and explosions ring out over the city during the attack.
The air force said that Russia had attacked with 145 drones, including the Iranian-designed Shahed, and that 85 were downed by air defence units.
Officials posted images of rescue workers in Mykolaiv evacuating the wounded from the scene of the attack and digging through the debris.
Apart from the matter of its armed forces, Ukraine has said it will never recognise Russian sovereignty over Ukrainian land, but it has publicly acknowledged that it will not be possible to recover some of the territory by force.
In addition to maintaining a strong Ukrainian army, Mr Palisa emphasised the importance of security guarantees from Ukraine's US and European partners that could help deter future Russian attacks.
The United States has not committed to providing security guarantees, though some European powers have discussed a so-called 'coalition of the willing' that could deploy a troop contingent to bolster deterrence.
Mr Palisa said he expected Russia to step up its assaults later this month and in May.
The official said he thought Russia would focus its offensive push on the eastern city of Pokrovsk, but could also push around the eastern fronts near Kupiansk and Lyman as well as the southeastern fronts of Zaporizhzhia and Novopavlivka.
"The absolute priority, in my opinion, will now be on the Pokrovsk direction," he said.
Russian forces have been trying to encircle the strategically important city of Pokrovsk to advance its goal of taking full control of Ukraine's Donetsk region.
Russia already controls around a fifth of Ukrainian territory.
Ukraine meanwhile faces uncertainty over the future of US military aid.
A lack of new aid could in time impact the supply of missiles for its Patriot air defence systems, as well as its offensive HIMARS and ATACMS systems.
Mr Palisa said earlier this month that Ukraine had not yet discussed additional aid with the US, but that the issue could be raised as the ceasefire negotiations continue.
Ukraine is investing resources in developing its own air defences, he said, adding that several nations had agreed to transfer licences and technical documentation for the production of "certain equipment" in the country.
"The process is moving, and quite dynamically," he added.
