DAMASCUS, Syria —Clashes intensified Thursday between Syrian troops and insurgents in central and northwestern Syria, part of what a top general called a “wide-ranging” offensive aided by Russian airstrikes and apparently aimed at clearing positions near government strongholds on the coast.

U.S. defense officials said as many as four of the 26 long-range cruise missiles that Russia said Wednesday it fired at Syria landed instead in Iran, but it was unclear if they caused any significant damage.

Russia’s involvement in Syria, which began with airstrikes Sept. 30 and escalated Wednesday with cruise missiles, “raises serious concerns,” NATO Secretary-General Jens Stoltenberg said after a meeting of the alliance’s defense ministers in Brussels.

Russia says its air campaign targets militants of the Islamic State and al-Qaida-linked groups, but the West accuses it of intervening to support President Bashar against even moderate rebels.