Russia announced it will pull the bulk of its troops from Syria starting March 15, in a process that could take up to 5 months. (Ministry of Defence of the Russian Federation) Ministry of Defence of the Russian Federation Just under half of Russia's fixed-wing strike force based in Syria has flown out of the country in the past two days, according to a Reuters calculation which suggests the Kremlin is accelerating its partial withdrawal.

President Vladimir Putin on Monday ordered the bulk of the Russian military contingent in Syria to be pulled out after five months of air strikes, saying the Kremlin had achieved most of its objectives.

The precise number of planes Russia kept at its Hmeymim base in Syria's Latakia province is secret. But analysis of satellite imagery, air strikes and defense ministry statements suggested it had about 36 fixed-wing military jets there.

At least 15 of those planes have flown out in the past two days, a Reuters analysis of state television footage shows, including Su-24, Su-25, Su-30 and Su-34 jets.

Reuters could not independently verify the movements of the aircraft and it was impossible to determine whether other aircraft were flying into Syria to replace those that left.

Military analysts say the departing Su-24 and Su-25 planes, aging Soviet-era planes that have undergone some modernization, have been the workhorses of Russia's Syria campaign.

They carried out 75-80 percent of the more than 9,000 sorties flown by Russian pilots, said Maksim Shepovalenko, a former Russian military officer who is now deputy director of the Moscow-based Centre for Analysis of Strategies and Technologies (CAST).

A Russian jet heads back to Russia after a deployment in Syria. Минобороны России

Russian television has shown four Su-25 and five Su-24 jets leaving in the past two days. Russia is thought by defense analysts to have had 12 of each in Syria. Five Su-34s, and one Su-30 have also been seen leaving.

John Kirby, a spokesman for the US State Department, told reporters on Tuesday he did not have exact details of the Russian military contingent in Syria but said Moscow had "dozens of aircraft" based there.

Ruslan Pukhov, CAST's director, said he thought Russia would have to pull out at least half of its strike force for its partial withdrawal to be regarded as genuine.

"Otherwise people, both nationally and especially internationally, will say this is not really true and it is simply a regrouping," he told Reuters.

Heightened sensitivity

Russian servicemen watch Mi-28 military helicopters of the Berkuti (Golden Eagles) aerobatic team fly during the opening of the Army-2015 international military forum in Kubinka, outside Moscow, Russia, June 16, 2015. REUTERS/Maxim Shemetov

Russian sensitivity about foreign interest in the return of military equipment from Syria has heightened in recent days.

Russian news portal lifenews.ru on Wednesday reported the detention of an American aviation blogger it described as "a spy." It said he had been caught snooping around the Chkalovsky military airport north of Moscow. He was later released.

A source in the Russian security service told local media on Tuesday that two British diplomats had this month been caught covertly filming the Mozdok military air base in southern Russia. The British Foreign Office said the diplomats had been carrying out routine travel and had undergone all the necessary Russian checks.

Russia is known to maintain at least 14 military helicopters in Syria as well as fixed-wing reconnaissance drones. The helicopters, if withdrawn, are likely to be returned to Russia by air.

Russian officials have made clear that two Russian military bases will remain in Syria, as will a smaller strike force of infantry, armor and helicopters. Russia's advanced S-400 air defense missile system also looks likely to remain.

A frame grab shows a S-400 defense missile system at Hmeymim airbase Thomson Reuters

US officials have spoken of Russia having "a few thousand troops" in Syria. A Russian military source told the Interfax news agency that around 1,000 troops would stay, of whom more than half would be military advisers.

Andrey Frolov, a defense analyst at CAST, said Russia would leave behind "several" Su-30 and Su-35 jets.

Alexander Kots, a military correspondent who has worked in Syria for the pro-Kremlin Komsomolskaya Pravda newspaper, says he has been told that Russia could return its entire air strike force to Syria within just 48 hours.

Heavy equipment and armor would be evacuated by sea, he said. Some of it might also be warehoused in Syria or handed over to the Syrian army, he said.

If necessary, Russia is still able to swiftly come to President Bashar al-Assad's aid by deploying long-range bombers based in Russia or by firing cruise missiles from the Caspian Sea. It also has a naval force in the Mediterranean.

A Russian navy ship in the Caspian sea fires cruise missiles at targets in Syria. Минобороны России

According to the database of the Bosphorus Naval News project, which publishes photos of warships crossing the straits, more than a dozen Russian military vessels, including landing and missile ships and auxiliary vessels are likely to be in the Mediterranean right now.

That estimate is partly borne out by information from the authorities and publicly available shipping records.