Cutting-edge drones developed by Kyiv since Russia invaded more than four years ago have repeatedly targeted Russian ships in the Black Sea, including warships and oil tankers.
Ukraine's uncrewed fleet has succeeded in limiting the movements of Russia's once-dominant Black Sea navy, Kyiv officials say, in one of the country's biggest accomplishments of the war.
But Ukrainian officials reckon that Russian President Vladimir Putin is bent on prosecuting the invasion, despite his bigger army's slow and costly progress on the battlefield and US diplomatic efforts to find a settlement.
Putin was planning "an additional rapid mobilisation of several hundred thousand Russians by the end of the year", Zelenskiy said on social media late on Tuesday, citing Ukrainian intelligence reports.
Those reports have also said recently that Russia is expanding production of ballistic missiles and jet-powered drones, both of which are hard for Ukrainian air defences to counter.
Meanwhile, Ukraine is also trying to scale up its weapons manufacturing through agreements with partner countries, as both countries apparently plan to fight on.
The Ukrainian military "carried out a unique operation targeting the naval base in Novorossiysk - the last major stronghold of the Russian fleet in the Black Sea", Zelenskiy said in a social media post Wednesday.
The attack struck air defences, piers and other port infrastructure in the Krasnodar region city, he said.
Krasnodar regional governor Veniamin Kondratyev said that hundreds of Ukrainian drones attacked Novorossiysk, Anapa and Gelendzhik and the Temryuk district of the region overnight.
Russian air defences downed more than 500 Ukrainian drones, the Defense Ministry in Moscow said.
In Novorossiysk, an eight-year old child was killed in the attack, and a person wounded when his house in the Temryuk region was hit later died in hospital, Kondratyev said.
He said 12 other people in the region were wounded.
Debris from downed drones fell on the territory of four industrial plants and 21 residential buildings, according to Kondratyev, but officials gave no details of any damage.
Novorossiysk is also home to a key international oil terminal.
The Grushovaya oil trans-shipment complex near Novorossiysk is one of the largest trans-shipment hubs in southern Russia for oil and petroleum products.
In February, the US State Department expressed its displeasure about Ukraine's attacks on Novorossiysk, saying they have affected US oil interests in Kazakhstan.
The Caspian Pipeline Consortium operates a pipeline from the Caspian coast in northwest Kazakhstan to Novorossiysk.
The pipeline handles much of the crude exports from three major Kazakh fields in which major US energy companies, Chevron and ExxonMobil, have stakes.
In illegally annexed Crimea, air defences downed 115 Ukrainian drones over Sevastopol overnight, the city's Russia-appointed head Mikhail Razvozhayev said, adding that 10 apartment buildings and 25 private houses were damaged.
Meanwhile, Ukraine's air force said Russia fired 138 long-range strike drones as well as an unspecified number of missiles at Ukraine during the night.
Damage was recorded at 16 locations across the country, it said.
In the southern Kherson region of Ukraine, a large-scale Russian drone attack overnight killed two people and wounded two others, regional head Oleksandr Prokudin said on Wednesday.
The attacks also destroyed a shopping mall in the southern Ukraine city of Zaporizhzhia, according to authorities.
