DAMASCUS — Syrian troops and allied militia backed by a fresh wave of Russian air strikes and cruise missiles fired from warships attacked rebel forces on Thursday as the government extended an offensive to recapture territory in the west of the country.

The assault focused on western areas where rebel advances earlier this year had threatened the coastal region vital to President Bashar al-Assad’s support base.

The Russian Defense Ministry said it fired missiles from ships in the Caspian Sea for a second day and had hit weapons factories, arms dumps, command centers and training camps.

U.S. officials said they believed four Russian cruise missiles bound for Syria had crashed en route in Iran. Russia’s Defense Ministry insisted the missiles had reached their targets in Syria.

The White House declined to comment and State Department spokesman John Kirby said he could not confirm the missiles had crashed, while adding that the report pointed towards the need for procedures to prevent clashes with U.S. planes targeting ISIS militants in Syria.

Since Russia began air strikes last week it has described all its targets as belonging to ISIS, although most have been in areas controlled by other rebel movements where ISIS has little or no presence.

On Thursday, Moscow said its air force hit 27 ISIS targets in the provinces of Homs, Hama and Raqqa, while Kirby said Secretary of State John Kerry had expressed his concern to Russian Foreign Minister Sergei Lavrov by telephone that most of the targets hit so far were not related to ISIS.

“There is an attempt by the regime to advance but the situation is under our control,” said a fighter in the Ghab Plain area in the west  from the Ajnad al-Sham insurgent group who uses the name Abu al-Baraa al-Hamawi.

The Syrian Observatory for Human Rights, which monitors the conflict, said rebels had shot down a helicopter in Hama province in western Syria. It was unclear if it was Syrian or Russian.

Sources in the region say Iran has sent hundreds of troops to back Syrian forces in a ground campaign coordinated with Russia’s air assault. Assad’s government also relies on support from Hezbollah.