KYIV, Ukraine -- Ukraine is open to considering proposals to place a ballistic missile-defence system on its territory to ward off the risk of attacks from Russia, a senior Ukrainian defence official said Wednesday. So far no one has offered.

Oleksandr Turchynov, the head of Ukraine's National Security Council, told Ukrinform news agency in an interview that Russia has become an increased threat since annexing the Black Sea peninsula of Crimea and increasing its military presence there.

Russian news agencies cited Kremlin spokesman Dmitry Peskov as saying Wednesday that the deployment of a missile-defence system in Ukraine would force Russia to adopt countermeasures.

In Washington, State Department spokeswoman Marie Harf said "There's no offer or plan to place U.S. or NATO ballistic missile defence systems in Ukraine. I don't think we're exactly sure what they're referring to."

Ukraine has repeatedly raised alarms about what it sees as Russia's aggressive military posture. It says Moscow has actively supplied separatists in east Ukraine with arms and manpower and that it routinely bolsters offensive capabilities in western Russia.

President Petro Poroshenko's government is concerned that Russia is making concerted efforts to move its nuclear capabilities to Crimea, which was absorbed by Moscow in 2014 following a referendum almost universally rejected by the international community.

"That the annexation of Crimea has significantly increased Russia's military capabilities and changed its balance of military power in the Black Sea and Mediterranean is understood by all our partners," Turchynov said. "But nobody goes beyond issuing statements and expressing deep concern."

"Ten Iskander-M tactical missile systems have already been delivered to the peninsula near the village of Shcholkine and Krasnoperekopsk," Turchynov told Ukrinform.

Russian Defence Ministry officials have also said they will deploy long-range, nuclear-capable Tu-22M3 bombers to Crimea.

Turchynov suggested that the West should consider improving its own security by barring Russian warships from passing through the Bosporus Strait -- the narrow stretch that divides Turkey's European and Asian parts and links the Black Sea and the Mediterranean.

NATO's U.S.-led missile-defence plans envisage deploying elements of the missile shield around Europe for what it says would be defence against Iran. Moscow sees this as a threat to its nuclear deterrent.

Unrest in eastern Ukraine has diminished markedly since a cease-fire was agreed between the government and Russian-backed separatist forces in February, but skirmishes continue daily. Military spokesman Andriy Lysenko said Wednesday that three government troops had been killed in action over the previous day.

Poroshenko said at a press conference in Kyiv with Slovakian counterpart Andrej Kiska that Russia has between 4,000 and 14,000 soldiers stationed in Ukraine at any given time.

"This is not just Ukrainian information. This is intelligence from NATO countries and other sources," Poroshenko said.

Ukrainian authorities this week showed off two men they say are Russian soldiers who were captured while on active duty in the rebellious east.

In video statements posted by the Ukrainian Security Service, the men say they were taking part in a reconnaissance operation in the Luhansk region Saturday when they were fired on, wounded and captured. Both say they were members of an army brigade based in the Russian city of Togliatti and had been deployed in Ukraine for more than a month.