NATO has rejected Russia's explanation that its warplanes violated the airspace of alliance member Turkey over the weekend by mistake and said Russia was sending more ground troops to Syria.
The comments by the alliance on Tuesday came as Turkish President Recep Tayyip Erdogan said he was losing patience with Russian violations of his country's airspace, and Russian officials said they would welcome talks with their Turkish counterparts to avoid "misunderstandings".
"An attack on Turkey means an attack on NATO," Erdogan said at a Brussels news conference.
Jens Stoltenberg, NATO secretary-general, 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 US warplanes fly combat missions over the same country for the first time since World War II.
The Russian defence ministry had said that an SU-30 fighter jet had entered Turkish airspace along the border with Syria "for a few seconds" on Saturday, a mistake caused by bad weather.
NATO says a plane also entered Turkish airspace on Sunday, an incident Russia says it is looking into.
Stoltenberg declined to comment on whether the Russian planes had locked their radar on the F-16 Turkish jets scrambled on Saturday to remove Russian aircraft from the airspace, usually a prelude to firing.
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Anatoly Antonov, Russian deputy defence minister, said Russia would welcome a Turkish defence ministry delegation to discuss avoiding any "misunderstandings" in Syria.
On Tuesday, Russian jets hit ISIL targets in the Syrian city of Palmyra and the northern province of Aleppo, Syrian state television and a monitoring group said.
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The strikes destroyed 20 vehicles and three weapon depots in ISIL-held Palmyra, state television said, quoting a military source.
In Aleppo, they hit the towns of al-Bab and Deir Hafer, about 20km east of a military airport currently besieged by ISIL fighters.
The Syrian Observatory for Human Rights, a Britain-based group tracking Syria's conflict, said the Palmyra strikes killed 15 ISIL fighters.
ISIL forces captured Palmyra, known as Tadmur in Arabic, 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 their control.
Syria's antiquities chief, Maamoun Abdulkarim, said on Sunday that ISIL fighters blew up Palmyra's Arch of Triumph, one of the most treasured monuments in the 2,000-year-old city. They had already destroyed temples and other antiquities.