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.

A Russian navy ship launches a cruise missile from the Caspian Sea (AP)

A still image taken from footage of an attack made by a Russian fighter jet in Syria

A Russian warship of the Caspian Flotilla launching missiles from the deployment area in the south-western Caspian Sea (EPA)

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.

In pictures: Russian air strikes in Syria Show all 19 1 /19 In pictures: Russian air strikes in Syria In pictures: Russian air strikes in Syria Syrian boys cry following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Russian defense ministry spokesman Major General Igor Konashenkov speaks to the media in Moscow, Russia. Konashenkov strongly warned the United States against striking Syrian government forces and issued a thinly-veiled threat to use Russian air defense assets to protect them AP In pictures: Russian air strikes in Syria Syrians wait to receive treatment at a hospital following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Alepp Getty In pictures: Russian air strikes in Syria Russian Deputy Defense Minister Anatoly Antonov speaks at a briefing in the Defense Ministry in Moscow, Russia. Antonov said the Russian air strikes in Syria have killed about 35,000 militants, including about 2,700 residents of Russia AP In pictures: Russian air strikes in Syria Jameel Mustafa Habboush, receives oxygen from civil defence volunteers, known as the white helmets, as they rescue him from under the rubble of a building following Russian air strikes on the rebel-held Fardous neighbourhood of the northern embattled Syrian city of Aleppo Getty In pictures: Russian air strikes in Syria Civil defence members rest amidst rubble in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria A girl carrying a baby inspects damage in a site hit by what activists said were airstrikes carried out by the Russian air force in the town of Douma, eastern Ghouta in Damascus, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members look for survivors at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Civilians and civil defence members carry an injured woman on a stretcher at a site damaged after Russian air strikes on the Syrian rebel-held city of Idlib, Syria Reuters In pictures: Russian air strikes in Syria Volunteers from Syria Civil Defence, also known as the White Helmets, help civilians after Russia carried out its first airstrikes in Syria In pictures: Russian air strikes in Syria The aftermath of Russian airstrike in Talbiseh, Syria In pictures: Russian air strikes in Syria Smoke billows from buildings in Talbiseh, in Homs province, western Syria, after airstrikes by Russian warplanes AP In pictures: Russian air strikes in Syria Russian Air Forces carry out an air strike in the ISIS controlled Al-Raqqah Governorate. Russia's KAB-500s bombs completely destroy the Liwa al-Haqq command unit In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria Â© TASS/ITAR-TASS Photo/Corbis In pictures: Russian air strikes in Syria Russia claimed it hit eight Isis targets, including a "terrorist HQ and co-ordination centre" that was completely destroyed In pictures: Russian air strikes in Syria A video grab taken from the footage made available on the Russian Defence Ministry's official website, purporting to show an airstrike in Syria In pictures: Russian air strikes in Syria A release from the Russian defence ministry purportedly showing targets in Syria being hit In pictures: Russian air strikes in Syria Russia launched air strikes in war-torn Syria, its first military engagement outside the former Soviet Union since the occupation of Afghanistan in 1979. Russian warplanes carried out strikes in three Syrian provinces along with regime aircraft as Putin seeks to steal US President Barack Obama's thunder by pushing a rival plan to defeat Isis militants in Syria In pictures: Russian air strikes in Syria Caspian Flotilla of the Russian Navy firing Kalibr cruise missiles against remote Isis targets in Syria, a thousand kilometres away. The targets include ammunition factories, ammunition and fuel depots, command centres, and training camps Â© TASS/ITAR-TASS Photo/Corbis

A meeting of Nato leaders on Thursday was dominated by concerns over Russia's recent military actions in Syria.

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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.