Ukraine has accused Russia of removing weapons and equipment – including toilets – from navy boats it returned after seizing them in the Black Sea.
"[Russia] removed even dome lights, sockets, and toilet bowls," Ukraine's navy commander Ihor Voronchenko said.
But Russia's Federal Security Service (FSB) said the ships were returned in "normal condition and with good plumbing".
The boats, which were seized last November, were handed over on Monday.
The 24 Ukrainian sailors on board the ships were returned in September in a prisoner swap deal.
Video footage reportedly from inside one of the vessels appears to show a toilet in place and intact, but it is unclear when it was shot.
Ukraine's President Volodymyr Zelensky inspected the two gunboats, Berdyansk and Nikopol, and the Yana Kapa tug on Thursday at the Ukrainian port of Ochakiv, near the coastal city of Odesa.
According to Russian state news agency Tass, Mr Zelensky later told reporters that equipment was "partially missing".
"The admiral and I had a conversation, and some things cannot be disclosed, but part of the weaponry is missing," he said. "I noticed the condition and there will be an investigation. We will definitely contact the Russian side so that it returns everything that was there."
The FSB, however, said on Thursday that no complaints were made at the time of the handover, and that documents had been signed accepting the condition of the ships.
"If during the transition… the Ukrainian side managed to bring the plumbing to an inadequate state, then this is the problem of the Ukrainian side," it said.
A short video was later tweeted by Russian reporter Alexander Kots, who wrote: "Ahaha! And the toilet is in place."
Russian forces annexed Ukraine's Crimea peninsula, which has a Russian-speaking majority, in March 2014, drawing international condemnation. The seizure of the vessels a year ago as they attempted to pass through the Kerch Strait between Crimea and Russia led to a serious escalation in tensions.
When Russia agreed to return the ships, it was seen as marking an improvement in the relationship between the two countries.
How and why were the boats seized?
On the morning of 25 November 2018, the three vessels tried to sail from the Black Sea port of Odessa to Mariupol in the Sea of Azov.
Ukraine said that as they sailed towards the Kerch Strait, they were intercepted by Russians ships, which rammed the tug. The vessels continued for a short distance, but were then prevented from travelling further by a tanker.
Russian forces opened fire and several Ukrainian sailors were injured.
Ukraine called it an act of Russian aggression, saying the Black Sea was free for shipping and that annexed Crimea belonged to Ukraine.