Four cruise missiles fired on Syria from a Russian ship in the Caspian Basin have sunk to Iran on Thursday, a US official said on Thursday, while the Russian side immediately denied the news.

This American source as well as the media there said that four missiles fell to Iran. Russia, he said, fires cruise missiles across Iran in Syria and attacks on the targets of rebels who are fighting against President Bashar al-Assad.

The US State Department said it could not confirm these reports.

Russia has said that four missiles hit their targets.

"All our fired missiles hit their goal," spokeswoman of Russian Defense Ministry Igor Kjustenkov said.

NATO Secretary-General Jens Stoltenberg called on Thursday, "worrying escalation," Russia's military action in Syria, which is increasingly striking in support of the Bashar al-Assad regime and whose aircraft hurt the airspace of Turkey, a member of the North Atlantic Alliance.

As the Russian Foreign Ministry announced, Russian and US diplomacy chiefs Sergey Lavrov and John Kerry discussed on Thursday the situation in Syria. They discussed co-ordination of the struggle against the Islamic state and the need to avoid incidents in the Syrian airspace where Russian and American aircraft operate.

The United States has announced on Wednesday that it will not cooperate militaryly with Russia in Syria because Moscow's strategy is "tragically deficient," as US Secretary of Defense Ash Carter said, while the Russian Defense Ministry announced that the United States is only seeking an excuse not to cooperate in the fight against terrorism .

However, the US stressed that it is willing to maintain basic, technical talks with Russia in order to try to secure the safety of pilots.