At least four of the 26 long-range cruise missiles fired from a Russian warship towards targets in Syria have crashed in Iran, according to US officials.
At least four missiles fell as they flew over Iran, two officials said citing military and intelligence information.
It remains unclear whether anyone was injured in the reported incident on Wednesday, or where the rockets landed in Iran.
However, it is likely the bombs hit a north-western region, as Russia has sent missiles headed for Syria across the area and over Iraq.
In response, the Russian Ministry of Defense said, somewhat sarcastically: "However unpleasant and `unexpected' it may be for our colleagues at the Pentagon and Langley about yesterday's attacks by high-accuracy weapons on the (Islamic State) infrastructure in Syria - all the same, all rockets fired from ships found their targets," Maj. Gen. Igor Konashenkov said.
The semi-official Fars news agency on Thursday said Western news reports about missiles going astray amounted to U.S. "psychological warfare" against Russia's intervention in Syria. An earlier report by Fars on Wednesday quoted Iraj Saghafi, acting governor of Takab in northwestern Iran, saying an explosion heard in the region was "possibly related to work in a nearby rock quarry."
On Wednesday, the Russian government said it had launched 26 cruise missiles, which successfully hit targets in north and northwest Syria. However, it did not say that any missiles went astray.
Russia stepped up its military campaign in Syria on Wednesday, when it began directing missiles from warships in the Caspian Sea.
The reported crash happened a week after Moscow started bombing Syria from the air.
Russia's military intervention in Syria, with which it is allied, has sparked deep concern among world leaders.
It is feared that Moscow is helping President Bashar al-Assad weaken anti-government rebels under the guise that it is predominantly attacking Isis, which has claimed swathes of the war-torn country since last summer.
A separate US-led coaltion against Isis, which is targeting the group with airstrikes in Iraq and Syria, is also a cause for concern due to the prospect of accidental clashes.
The US state department fuelled worries on Wednesday, by claiming that more than 90 percent of Russia airstrikes in Syria were not directed at Isis or al-Qaida groups.
Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry discussed the Syrian conflict during a phone call on Thursday, Russia's Foreign Ministry said in a statement.
The counterparts discussed coordination in the fight against Islamic State and the necessity to avoid incidents in Syrian airspace, the ministry said.
A meeting of Nato leaders on Thursday was dominated by concerns over Russia's recent military actions in Syria.
The alliance announced that it had finalised plans for a response force of up to 40,000, twice the current size.
Jens Stoltenberg announced the ministers' decisions at a news conference.
“All of this sends a clear message to all Nato citizens. Nato will defend you, Nato is on the ground, Nato is ready,” Mr Stoltenberg said.
Additional reporting by AP, PA and Reuters
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