Nato Secretary General Jens Stoltenberg was in Kyiv on Thursday in an unannounced visit, on which he pledged to "continue strengthening Ukraine's armed forces".
He met Ukrainian President Volodymyr Zelenskyy on his first visit to the country since Russia launched its invasion last year.
The US-led military alliance "stands with you today, tomorrow and for as long as it takes,” said Mr Stoltenberg.
Mr Zelenskyy hailed it as a sign of closer relations with Nato, which Ukraine hopes to one day join.
Ukrainian officials published images of Mr Stoltenberg in central Kyiv at a memorial for fallen soldiers.
He laid a wreath to honour troops who died during the fighting in Ukraine's east, and inspected captured Russian vehicles on Kyiv's St Michael's Square.
Mr Stoltenberg arrived in Ukraine early on Thursday. Many western officials travelling to Kyiv do not announce their trips in advance for security reasons.
Nato have sent Ukraine large shipments of tanks, rocket launchers, ammunition and other military equipment to resist Russia's advance.
"Allies are now delivering more jets, tanks and armoured vehicles, and Nato's Ukraine fund is providing urgent support ... all of this is making a real difference on the battlefield today," Mr Stoltenberg said.
Ukraine's allies will hold another round of talks at a US airbase in Germany on Friday to discuss military aid.
"The further our soldiers can move on the front line, the sooner the most important thing — peace — will return to Ukraine and all of Europe,"
On the question of Ukraine's possible future membership, Mr Stoltenberg said Ukraine's "rightful place is in Nato" but that "multiyear support" was needed to bring a former Soviet military to Nato standards.
Ukraine launched its bid for fast-track Nato membership in September, after Moscow announced the purported annexation of four Ukrainian regions.
Russia on Thursday said preventing Ukraine from joining Nato remained one of the main aims of its 14-month invasion.
"Otherwise, it will pose a serious danger to our country, to its security," the Kremlin said.
Any accession would need to be approved by all 31 existing allies. Finland joined Nato this month, with Sweden expected to follow soon.
Nato leaders including US President Joe Biden will meet for an annual summit in the Lithuanian capital Vilnius in July.
The alliance has "no alternative" but to support Kyiv, Czech President Petr Pavel said, after a meeting with Mr Stoltenberg on Wednesday.
"The will in Europe to continue supporting Ukraine is enormous," Mr Stoltenberg said in a joint press conference.
