A photo of a Russian warship has raised concerns amid mounting tensions between Russia and Ukraine.
Two Russian warships transited the Bosphorus en route to the Black Sea on Saturday and 15 smaller vessels completed a transfer to the sea as Moscow beefs up its naval presence at a time of tense relations with the West and Ukraine.
The reinforcement coincides with a huge build-up of Russian troops near Ukraine, something Moscow calls a temporary defensive exercise, and follows an escalation in fighting in eastern Ukraine between Russian-backed separatists and Ukrainian government forces.
Russia's relations with Washington, which cancelled the deployment of two of its own warships to the Black Sea last week after fierce Russian protests, are at a post-Cold war low.
Moscow expelled 10 US diplomats on Friday in retaliation for the expulsion of the same number of Russian diplomats from the United States over alleged malign activity.
Russia has also temporarily restricted the movement of foreign warships "and other state ships" near Crimea, which it annexed from Ukraine in 2014, a move condemned by both Kyiv and Washington.
Two Russian Ropucha-class landing ships from Russia's Northern Fleet, capable of carrying tanks and of delivering armour and troops during coastal assaults, transited the Bosphorus on Saturday, a Reuters reporter in Istanbul saw.
More Russian naval reinforcements in the form of two more landing ships, this time from Russia's Baltic Fleet, are expected to imminently transit the Bosphorus.
The RIA news agency on Saturday also reported that 15 smaller vessels from Russia's Caspian Flotilla have completed their transfer to the Black Sea as part of an exercise.
In a further sign of heightened tensions in the region, a ship carrying logistics trucks and equipment for NATO forces in Romania transited the Bosphorus on Friday evening, the same Reuters reporter saw.
British warships to head to the Black Sea
British warships will also sail for the Black Sea in May amid the rising tensions, the Sunday Times newspaper reports, citing senior naval sources.
The deployment is aimed at showing solidarity with Ukraine and Britain's NATO allies, the newspaper reported.
One Type 45 destroyer armed with anti-aircraft missiles and an anti-submarine Type 23 frigate will leave the Royal Navy's carrier task group in the Mediterranean and head through the Bosphorus into the Black Sea, according to the report.
RAF F-35B Lightning stealth jets and Merlin submarine-hunting helicopters will stand ready on the task group's flag ship, the carrier HMS Queen Elizabeth, to support the warships in the Black Sea, the report added.
Officials at the UK Ministry of Defence were not immediately available for comment.
A ministry spokesman told the newspaper that the UK government was working closely with Ukraine to monitor the situation and continued to call on Russia to de-escalate.
"The UK and our international allies are unwavering in our support for Ukraine's sovereignty and territorial integrity," the newspaper quoted the spokesman as saying.
Ukrainian diplomat to be expelled
Russia on Saturday said it would also expel a Ukrainian diplomat, prompting an immediate pledge of retaliation from Kiev, further escalating tensions over Moscow's troop build-up on Ukraine's eastern flank.
Moscow claimed the Ukrainian diplomat had been caught "red-handed" trying to obtain sensitive information.
In response, the Ukrainian foreign ministry protested "the illegal" detention of its diplomat and said Kiev would expel a senior Russian diplomat.
Kiev has been battling Russian-backed separatists in eastern Ukraine since 2014 and clashes intensified early this year, effectively shredding a ceasefire agreed last July.
Around 30 Ukrainian troops have been killed since the start of the year, compared to 50 last year. Most of them were victims of sniper fire.
Russia has detained a number of Ukrainian nationals on suspicion of spying in recent years but the arrest of a diplomat is rare.
"A Ukrainian diplomat, a consul of Ukraine's Consulate General in Saint Petersburg, Alexander Sosonyuk, has been detained by Russia's FSB," the security service said in a statement, using the Russian language spelling of his name.
He was held on Friday, the security service said. Kiev said the diplomat had spent several hours in detention.
The Russian foreign ministry said Saturday it had summoned Ukraine's charge d'affaires Vasyl Pokotylo and told him that the Ukrainian diplomat had 72 hours to leave the country "beginning April 19".
"The Russian side pointed to the inadmissibility of such kind of activity," the foreign ministry said in a statement.
Russia's domestic intelligence agency said Sosonyuk had been caught "red-handed" during a meeting with a Russian national as he sought to receive "classified" information.
"Such activity is not compatible with his diplomatic status and is clearly hostile in nature towards the Russian Federation," the FSB said.
with AFP and Reuters
Do you have a story tip? Email: firstname.lastname@example.org.