The Russian Navy's large landing ship Korolev sails in the Bosphorus, on its way to the Mediterranean Sea, in Istanbul, Turkey, in this October 1, 2015 file photo.  REUTERS

Moscow/ Reuters

A ship chartered by the Russian government to make voyages to a government-controlled port in Syria was carrying military trucks when it headed to Syria last month, according to photographs taken as it passed through the Bosphorus Straits.

The photographs, taken by a Turkish blogger who passed them to Reuters, follow a Reuters report that Russia has set up a seaborne lifeline via the Bosphorus to supply its armed forces in Syria and President Bashar al-Assad's forces as it steps up its involvement in the conflict.

The photographs come at a sensitive time in relations between Russia and Turkey after two Russian warplanes strayed into Turkish airspace at the weekend, angering Ankara and prompting threats of retaliation.

Reuters reported last month that a Russian ferry called the Alexandr Tkachenko had docked at the Syrian port of Tartous on Sept. 11, where Moscow leases a naval facility, after stopping off to pick up cargo at the Russian port of Novorossiisk.

Its journey was part of a significant uptick in merchant shipping heading for Syria from Russia; it was one of six ships that made their way from Novorossiisk to Syria within two weeks in mid-September.

On Sept. 30, Russia launched its first air strikes in Syria.

 The Tkachenko transited from the Black Sea into the Mediterranean via the Bosphorus, passing Istanbul in the process.

A port source said at the time that the vessel had been carrying white-painted trucks. But photographs supplied by an Istanbul-based blogger and shipping enthusiast, Yoruk Isik, clearly show the same vessel, the Tkachenko, carrying Russian army Ural-4320 military trucks on its main cargo deck as it passed through the Bosphorus en route to Syria on Sept. 6.

The Ural-4320, a workhorse for the Russian army, is used to transport weapons and troops. It can also be used as a platform for a rocket launcher.

The images show two such trucks on the deck of the ageing yellow vessel. Partially obscured by what look like fuel storage units, the tops of what look like three other unidentified military trucks can also be seen.

Reuters authenticated the images by cross-checking the date and location details with its own data and by ensuring the photos had not been tampered with.

 Russian shipping

An unnamed employee of the company that had previously been using the Tkachenko in Crimea said the Russian government had later chartered it. The Russian defence ministry did not respond to written questions from Reuters about whether it had done so.

Isik, the Turkish blogger, said he had noticed a surge in Russian shipping in late summer with Russian naval landing ships loaded up with what looked like trucks and armoured vehicles carefully concealed with tarpaulins.

"Being from Istanbul, I'm used to Russian navy ships," Isik told Reuters.

"They passed very regularly in 2014 and the first half of 2015. From August, the traffic reached a level I've never witnessed before."

He said he saw the Tkachenko pass through the Bosphorus twice in September. The second time was at night, making it harder to discern what it was carrying, though he believes its cargo was prefabricated housing units.

U.S. officials have said such units have appeared at an air base in Syria's Latakia province from where Russia is conducting its air campaign against Islamist militants.

Port arrival data confirms the Tkachenko's second trip to Tartous. It was one of three vessels that made their was from Russia to Syria in October.

Last week, the Antigua and Barbuda-flagged Atlantic Prodigy from Russia's Novorossiisk docked at the Syrian port of Latakia. And on Tuesday, another vehicle carrier, the Palau-flagged Novorossiisk, passed Cyprus and is now approaching Tartous.

There is also growing evidence that Russia is using its giant Antonov-124 aircraft to fly military supplies into Syria, including possibly fighter planes.

Publicly available flight tracking systems have spotted the same transport planes in Siberia, close to a factory where the Sukhoi-34 warplanes are made, and in Syria at least once. Sukhoi said on Tuesday it had delivered a new batch of the strike fighters to the defence ministry.

A source familiar with the contract details said the batch included three planes, part of 12 due to be delivered this year.

The same Antonov transport planes have also been spotted in Syria and at an air base in southern Russia, near the town of Krymsk.

Isik, the Turkish blogger, says his vantage points means he is ideally placed to track Russian shipping to Syria, a hobby he says he intends to continue.

"Because of Istanbul's unique geographical position, the events of nearby conflicts somehow unfold in front of our eyes here on the Bosphorus."