US Defence Secretary Ashton Carter on Thursday predicted Russia would soon begin suffering casualties in Syria, as Moscow dramatically ramps up its military campaign in the devastated nation.
Russia began bombing in Syria last week, radically changing the course of the four-and-a-half-year conflict and providing a massive boost to the regime of President Bashar al-Assad.
But the military campaign "will have consequences for Russia itself, which is rightly fearful of attacks," Carter said.
"In the coming days, the Russians will begin to suffer from casualties."
In a major development, Russia fired cruise missiles from the Caspian Sea on Wednesday as cover for a Syrian army ground offensive.
Carter said Moscow has been reckless in its military commitment, risking clashes with US and other planes targeting Islamic State (IS) jihadists in Syria as part of a 60-plus member coalition.
"They have shot cruise missiles from a ship in the Caspian Sea without warning; they have come within just a few miles (kilometres) of one of our unmanned aerial vehicles," Carter said.
NATO diplomatic sources said Moscow gave no advance warning of the cruise missile strikes, which came as a complete surprise.
A Syrian general said the Russian intervention had weakened IS and other opponents of Assad, but Washington insists the vast majority of Moscow's strikes have targeted Western-backed moderate opposition groups.
"There's a pattern of saying one thing and doing another" by the Russians, Carter said.
"They said they were going to fight (Islamic State) but that doesn't match up with the targets."
Russia has justified its intervention in Syria as part of global efforts to fight "terrorists" and also to target Russian jihadists who may one day attack their own country.
Like Carter, NATO Secretary General Jens Stoltenberg warned of a "troubling escalation" in Russian military activity in Syria.
"We will assess the latest developments and their implications for the security of the alliance," Stoltenberg said.
Russia's defence ministry said Thursday that its warplanes had conducted 22 flights overnight, striking "27 terrorist targets".
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- Air space violations -
In military manoeuvres reminiscent of Cold War brinkmanship, Russia has at least twice violated Turkey's air space, though Moscow claims this was accidental.
Before Russia's entrance into the war, NATO stationed anti-missile Patriot batteries in Turkey to protect it from any spillover of the Syrian conflict.
Some of these are due to be removed later this year for maintenance, but Spain and Germany are keeping their Patriots in Turkey.
Asked if NATO would now consider extending their mission in light of Russia's actions, Stoltenberg said: "NATO is ready and able to defend all allies against any threat, including Turkey."
The alliance has changed tack radically in the fallout from the Ukraine crisis after years of defence cuts, with leaders agreeing last year to increase spending and to set up a very rapid response force which should be operational from next year.
Carter stressed NATO should not be distracted from Ukraine by the Syria crisis, and should ensure Russia sticks to agreements made in Minsk last year.
- Afghanistan troop withdrawal -
NATO forces have been in Afghanistan since 2001 and member nations are worried about a planned draw-down of US troops by the end of next year, from about 10,000 to just 1,000.
Carter indicated the United States, by far the largest contingent of the mission, was open to modifying the withdrawal.
"I have asked all of the NATO partners to remain flexible and consider making adjustments to the plan, which is now two and a half years old," Carter said.
The NATO meeting is the main event of Carter's five-day trip to Europe, which also saw him visit Spain and Italy -- two ally nations dealing with emerging new threats on NATO's southern flank, such as political chaos or embassy attacks in North Africa. Both countries have US bases with fast access to the region.
The 61-year-old US defence chief has been in the Pentagon's top job since February and after a relatively low-key first few months now finds himself front and centre of the Syria crisis.
On Wednesday in Rome, he gave his strongest statement yet, dismissing Russian claims that Washington was open to coordinating military efforts in Syria.
Pentagon official stress the only communication with Russia about Syria is about accident-avoidance to ensure pilots from the coalition and Russia aren't in the same place at the same time.
On Friday, he heads to London for a brief visit with the British Defence Minister Michael Fallon.