WASHINGTON — Russia’s military buildup in Syria will probably prolong the life of the beleaguered government of President Bashar al-Assad, Pentagon officials and foreign policy experts say, but is unlikely to be a major factor in the campaign to defeat the Islamic State, and could further inflame — and lengthen — the conflict.

The arrival of four multipurpose warplanes at an airfield near Latakia, Syria, on Monday brought the number of tactical jets that Moscow has deployed to Syria this month to 32. They further enhanced Russia’s ability to carry out airstrikes that experts say can give Syrian government forces a badly needed boost on the battlefield. Reconnaissance flights by Russian drones in the last week have all been over areas controlled by opponents of Mr. Assad — some backed by the United States and its allies — while avoiding territory controlled by the Islamic State.

If Russia takes the next step of sharing the intelligence with the Syrian government or carrying out airstrikes against those groups, it could easily lead to an escalation in the conflict, frustrating already-dwindling hopes for a diplomatic resolution and prompting Arab governments to increase aid to Syrian rebels.

Adel al-Jubeir, Saudi Arabia’s foreign minister, said Tuesday that there were no circumstances in which his country would accept the Russian effort to keep Mr. Assad in power.