NATO chief Mark Rutte arrived in Kyiv on Wednesday for an unannounced trip, Ukraine's national railway operator said in a later deleted social media post.
It comes after a series of large-scale fatal Russian attacks on the Ukrainian capital in recent weeks.
Ukrainian President Volodymyr Zelenskyy, who is expected to meet with Rutte, has been appealing to NATO members for help protecting Ukraine from Russian ballistic missile attacks.
"Today at the Kyiv railway station, we are gladly welcoming NATO Secretary General Mark Rutte," Ukrzaliznytsia wrote in the now deleted post, alongside images of the NATO chief on the platform.
"This visit is extremely important, just like all the previous ones, because it is a gesture of solidarity and support from the Alliance for our country," Ukrzaliznytsia added.
His visit comes hours after Ukrainian drones hit energy and military sites in the northern Russian city of Saint Petersburg where officials and visiting dignitaries were gathering for a flagship economic forum.
Russian missile and drone attacks a day earlier killed 23 people in strikes on Kyiv and the eastern city of Dnipro.
