Russian fighter jets are on the ground in Syria, Pentagon officials told Fox News, in the latest escalation of Moscow’s military build-up there -- as the Obama administration seeks answers from the Putin government on their intentions.

In the first step in direct military talks proposed by Russia about the military buildup, Defense Secretary Ash Carter called his Russian counterpart on Friday. The 50-minute conversation marked the first time that American and Russian defense chiefs have spoken in more than a year amid U.S. anger over Russian invasion and annexation of Ukraine's Crimea region.

But shortly after the call, multiple officials confirmed to Fox News that Russian jets had landed in Syria, at Bassel al-Assad International Airport in Latakia, one of the last remaining strongholds of the Assad regime. Officials said they are Sukhoi Su-27s, one of Russia’s most modern fighter jets and capable of air-to-ground combat.

Moscow has been sending a steady stream of equipment and artillery in support of the Assad regime against the Islamic State.

Russian Minister of Defense Sergey Shoigu told Carter on Friday that the buildup of Russian forces in Syria was “defensive” in nature, a senior defense official said just minutes after the phone call took place.

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    The Obama administration has been flummoxed over how to respond to Russia's increased support for Syrian President Bashar Assad and has been worried it will interfere with the ongoing U.S. and coalition campaign against Islamic State militants in Syria.

    Pentagon press secretary Peter Cook said Carter and Shoigu had a constructive talk about the need to "de-conflict" the Russian buildup with the anti-Islamic State effort.

    Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov issued a statement noting that "the course of the conversation has shown that the sides' opinions on the majority of issues under consideration are close or coincide. The ministers noted the restoration of contacts between the countries' defense ministries and agreed to continue consultations."

    Secretary of State John Kerry told reporters in London that President Obama believes military-to-military discussions with Russia are "an important next step" as the U.S. and its allies seek to resolve a worsening Syria crisis while also trying to understand Russia's burgeoning role. Russian Foreign Minister Sergey Lavrov first broached the idea earlier this week in a phone call with Kerry.

    "This (Syria) crisis has got to be solved," Kerry said before meeting with the United Arab Emirates Foreign Minister Abdullah bin Zayed.

    Russia has called for a broad coalition to fight the Islamic State group and has indicated that helping Assad's military is the best way to do that. However, U.S. policy has centered for the duration of the more than four-year civil war in Syria that Assad must step down to make way for a new government. Until recently, Russia seemed to agree with that policy. But in recent weeks, it has become clear that Moscow intends to create an air operations base in Syria and has sent personnel, equipment and tanks to the coastal province of Latakia.

    And on Friday, a spokesman for Russian President Vladimir Putin said Russia would consider sending troops to fight in Syria if Damascus requested them.

    U.S. officials said in recent days that they are trying to glean the precise reasoning behind Moscow's recent deliveries.

    After their third phone call on the Syria situation since Labor Day weekend, Kerry said Wednesday that Lavrov had proposed a "military-to-military conversation and meeting in order to discuss the issue of precisely what will be done to de-conflict with respect to any potential risks that might be run and have a complete and clear understanding as to the road ahead and what the intentions are."

    Kerry said Lavrov had told him that Russia was only interested in confronting the threat posed by the Islamic State group in Syria. But Kerry stressed that that remained unclear.

    "Obviously, there are questions about that," he said. "I am not taking that at face value."

    However, Kerry added that if Russia's attention is on the Islamic State group, then it remains a potential partner in pushing for a political transition in Syria.

    Fox News’ Lucas Tomlinson and The Associated Press contributed to this report.