NATO's secretary-general on Tuesday rejected Russia's claim that its military incursion into alliance airspace over Turkey wasn't intentional or important, saying there were two separate incidents and "the violation lasted for a long time."

Jens Stoltenberg, secretary-general of the 28-nation military bloc, also said the alliance had reports of a substantial Russian military build-up in Syria, including ground troops and ships in the eastern Mediterranean.

Stoltenberg told a news conference in Brussels the reported airspace incidents were "very serious." He added: "It doesn't look like an accident, and we've seen two of them over the weekend." His comments come amid growing anger in Ankara over the move, with Turkish President Tayyip Erdogan warning Russia on Tuesday that it could lose its friendship over the issue.

"Our positive relationship with Russia is known. But if Russia loses a friend like Turkey, with whom it has been cooperating on many issues, it will lose a lot, and it should know that," Erdogan told a press conference in Brussels.

The Russian government is looking into the claim of a second violation of Turkish airspace, the Russian embassy in Ankara said on Tuesday, according to the TASS news agency. The Russian Defense Ministry said on Monday that an SU-30 fighter aircraft had entered Turkish airspace along the border with Syria "for a few seconds" on Saturday, a mistake caused by bad weather. On Tuesday, Russian Deputy Defense Minister Anatoly Antonov said he would welcome a Turkish delegation to talks to avoid "misunderstandings" in Syria.

The dispute between Turkey and Russia comes after Moscow’s intervention in Syria has effectively derailed Ankara’s desire to establish a buffer zone on its Syrian border. Turkey hoped a buffer zone would help stem the tide against ISIL and Kurdish factions that Ankara fears are exploiting the Syrian conflict to press for an independent Kurdish state in Turkey.

NATO generals, using pre-existing and dedicated military-to-military lines of communication with Moscow, would be contacting their Russian counterparts about the alleged entry of Russian warplanes into Turkish skies, NATO said in a statement.

"It's unacceptable to violate the airspace of another country," Stoltenberg told reporters. He said NATO is expressly worried that such acts by the Russians could have unforeseen consequences.

"Incidents, accidents, may create dangerous situations," Stoltenberg said. "And therefore it is also important to make sure that this doesn't happen again."

On Monday, NATO ambassadors met in special session and condemned what they termed Russia's "irresponsible behavior." The ambassadors also called on Russia to cease such practices.

A Turkish government official confirmed Tuesday that Russian Ambassador Andrey Karlov was called to the ministry on Monday afternoon, during which Turkish officials lodged a "strong protest" over the second infringement.

The incidents underscore the risks of an international escalation of the Syrian civil war, as Russian and U.S. planes fly combat missions over the same country for the first time since World War II.

Hours after the remarks my NATO's secretary-general, Russia told the U.S. that it is ready to resume military-to-military discussions aimed at keeping aircraft apart as they wage the parallel campaigns of airstrikes, according to a U.S official. U.S. Defense Secretary Ash Carter had earlier on Tuesday called on Moscow to urgently respond to proposed rules for air-to-air conduct over Syria.

The U.S., leading the coalition fighting the Islamic State in Syria and Iraq, or ISIL, wants to avoid being drawn into a proxy war with Russia, which is defending its ally, Syrian President Bashar Al-Assad.

Russian President Vladimir Putin has said he would not put ground forces in Syria, which is fighting a civil conflict that has killed some 250,000 people. However, Stoltenberg said there was a large 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.

Admiral Vladimir Komoyedov, the head of the lower house of the Russian parliament's defense committee, said on Monday that Moscow was not conducting operations in Syria involving its own ground troops and would not do so, according to the RIA Novosti news agency.

But a day prior, Komoyedov had said it was likely that Russian volunteers would travel to Syria to fight there, which many analysts and governments took as a potential code for deploying ground forces.

Meanwhile, in some of the heaviest attacks on ISIL since it began its ramped up operation last week, Russian jets hit targets in the Syrian city of Palmyra and the northern province of Aleppo, Syrian state television and a monitoring group said Tuesday.

The strikes destroyed 20 vehicles and three weapons 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 10 miles east of a military airport currently besieged by ISIL fighters.

The Syrian Observatory for Human Rights, a Britain-based, anti-Assad group tracking Syria's civil war, said the Palmyra strikes killed 15 ISIL fighters.

"It was the heaviest Russian attack on Palmyra," Observatory director Rami Abdulrahman said. Across Syria, Russian jets carried out at least 34 air strikes in the last 24 hours, the Observatory said.

Another four ISIL fighters were killed near Raqqa, the eastern city that has served as the group's stronghold in Syria for the last two years.

Al Jazeera and wire services