NATO on Tuesday rejected Moscow's explanation that its warplanes violated the air space of alliance member Turkey at the weekend by mistake and said Russia was sending more ground troops to Syria and building up its naval presence.
With Russia extending its air strikes to include the ancient city of Palmyra, Turkish President Tayyip Erdogan said he was losing patience with Russian violations of his country's air space.
"An attack on Turkey means an attack on NATO," Erdogan warned at a Brussels news conference.
NATO Secretary-General Jens Stoltenberg said the alliance had reports of a substantial Russian military build-up in Syria, including ground troops and ships in the eastern Mediterranean.
"I will not speculate on the motives ... but this does not look like an accident and we have seen two of them," Stoltenberg said of the air incursions over Turkey's border with Syria. He noted that they "lasted for a long time".
The incidents, which NATO has described as "extremely dangerous" and "unacceptable", underscore the risks of a further escalation of the Syrian civil war, as Russian and U.S. warplanes fly combat missions over the same country for the first time since World War Two.
The Russian Defense Ministry had said that an SU-30 warplane had entered Turkish air space along the border with Syria "for a few seconds" on Saturday, a mistake caused by bad weather. NATO says a plane also entered Turkish air space on Sunday, an incident Russia says it is looking into.
Separately, a U.S. official told Reuters the incursions lasted more than a few seconds and described Moscow's assertion that the incursions were an accident as "far-fetched".
Stoltenberg said the U.S.-led alliance had not received "any real explanation" from Russia about the incursions.
Air space violations
Disagreement over the air space violations came after disputes over the exact aims of the Russian air campaign. Moscow says it is attacking Islamic State but the West has accused it of striking other rebel groups to prop up its ally Syrian President Bashar al-Assad.
In a further incident, a MIG-29 fighter of unknown nationality and Syria-based missile systems "interfered" with eight Turkish F-16 jets patrolling along the Syrian border on Monday, the Turkish military said.
These are not the first such flashpoints. A Turkish jet was shot down by the Syrian air force in 2012 over the Mediterranean and earlier this year Turkey shot down a Syrian helicopter which Ankara said had violated its airspace.
Russia's NATO envoy said the alliance was using the accidental incursion to distort the aims of Moscow's air campaign in Syria, according to the TASS news agency.
"The impression is that the incident in Turkish air space was used to plug NATO as an organization into the information campaign waged by the West to distort the aims of the operations carried out by the Russian air force in Syria," Alexander Grushko was quoted as telling reporters in Brussels.
The United States, leading the coalition attacking Islamic State in Syria and Iraq, wants to avoid being drawn into a proxy war with Russia.
With that in mind, Russia's defense ministry said it agreed in principle with U.S. proposals on coordinating military flights in Syria. The ministry said it was ready to hold talks with Turkey to avoid "misunderstandings" over Syria and it invited foreign military officers to Moscow for talks on how best to fight Islamic State.
Russian President Vladimir Putin has said he will not put ground forces in Syria, where the civil war has killed 250,000 people. However, Stoltenberg said there was a growing presence of Russian forces in Syria.
"I can confirm that we have seen a substantial build-up of Russian forces in Syria: air forces, air defenses, but also ground troops in connection with the air base they have, and we also see an increased naval presence," Stoltenberg said.
U.S. officials have previously said Russia has sent seven T-90 tanks, some artillery, and about 200 marines. It has also deployed temporary housing units, a portable air traffic control station and components for an air defense system.
Russian defense ministry sources have been quoted in Russian media as saying about 1,500 Russian servicemen are involved in supporting the air strikes and supplying the Syrian army with equipment.
The Russian Defense Ministry itself has said it has more than 50 warplanes and helicopters in Syria. Russia's Tartous naval facility there is a logistics base and has been overhauled in recent years. It is being used to unload equipment, some of which is also being flown in.
Admiral Vladimir Komoyedov, the head of the lower house of the Russian parliament's defense committee, said Moscow was not conducting operations in Syria involving its own ground troops and would not do so, according to the RIA Novosti news agency.
Pressing ahead with an air campaign that began nearly a week ago, Russian jets hit Islamic State targets in Palmyra and the northern province of Aleppo, Syrian state television said, in some of the heaviest Russian attacks on the hardline group.
Islamic State forces captured Palmyra in May, an advance which brought them closer to the core of government-held territory in western Syria. It also put the city's Roman-era ruins under the militants' control.