Ukraine's maritime forces have been severely diminished by the Russian intervention in Crimea after they lost 12 of their 17 warships to Moscow, defence analysts Jane's said Wednesday.

Around 12,000 of the Ukrainian navy's 15,450 personnel were based in Crimea when Russia first intervened on February 27, and since then have lost the bulk of their assets, Jane's said in a commentary.

In the port city of Sevastopol, the Russians seized two Grisha V-class frigates, a patrol vessel and a command ship, as well as Ukraine's only submarine and an ocean-going tug.

Russian special forces also seized Ukraine's naval base at Novoozerne, on Crimea's northwest coast. Seven Ukrainian warships and a Natya-class minesweeper have been blockaded in the base on Lake Dunuzlov after Russia scuppered the hulks of two warships in the access channel leading to the sea.

Ukraine's 10th Saski Naval Air Brigade, which controlled all of the country's maritime aviation assets, managed to get some of its aircraft and helicopters onto bases on the mainland in time.

But Jane's said there will likely be problems caused by Moscow's seizure of Sevastopol Aviation Enterprise, which was responsible for the maintenance of Ukrainian naval helicopters.

Ukraine's navy is now based at Odessa, with less than half a dozen large surface craft as well as several small patrol craft, Jane's said.

But at least it has the Krivak III-class frigate Hetman Sagaidachny, which was returning from an anti-piracy mission in the Indian Ocean when the crisis broke.

"Ukraine's navy now faces an uncertain future," the commentary said.

"As well as losing the bulk of its ships, helicopters and aircraft, it headquarters building, much of the service's signals intelligence, training, administration, maintenance, and logistics infrastructure has now been lost.

"This includes the service's main underground ammunition storage site at Inkermann valley, outside Sevastopol."

Other military assets seized in Crimea include 39 Mig-29 fighters based at the Balbek airbase, as well as S-300 surface-to-air missiles from the Fiolent base.