By Dion Nissenbaum and Philip Shishkin / The Wall Street Journal

WASHINGTON — Russian forces have delivered a half-dozen tanks to a base in Syria, a marked escalation and the strongest indication yet that Moscow is preparing to transform a coastal airfield into a new military hub to help President Bashar al-Assad, U.S. officials said Monday.

Over the weekend, U.S. officials said, the six T-90 Russian tanks arrived at the airfield south of Latakia, Syria, where Russia’s military has embarked on an intensive buildup.

Along with the tanks, Russia has sent three dozen armored personnel carriers, about 15 new artillery pieces, and housing that could accommodate as many as 1,500 people, the officials said.

Russia is now flying an average of two cargo flights a day and the planes are using a new air route over Iraq, after Bulgaria rejected a request from Moscow last week to fly the cargo over its airspace, U.S. officials said.

The uptick in military deliveries, especially the tanks, has led to a shift in thinking at the U.S. Defense Department, where there is a growing consensus that Russia is preparing to play a much more intensive role in the Syrian conflict.

“Tanks are more offensive in nature, and this exceeds what we would consider as force protection for a base,” said one U.S. military official.

Russia has played down the importance of the military deliveries and cast them as part of its long-standing support for Mr. Assad. But U.S. officials are increasingly concerned about Moscow’s intentions.

President Barack Obama last week warned Russia that it was making a mistake in aiding Mr. Assad. Secretary of State John Kerry has called Russian Foreign Minister Sergei Lavrov to express America’s concerns. Officials at the Russian embassy in Washington didn’t immediately respond to questions Monday.

Russia has been one of Mr. Assad’s strongest international supporters, and Moscow will play a central role in steering the course of the conflict toward a potential diplomatic deal or a more deadly conflict.

In Syria’s multifaceted civil war, Washington has sought to isolate the Assad regime diplomatically while developing Syrian rebel forces that have so far proved largely ineffective on the battlefield.

On Monday, Capt. Jeff Davis, a Pentagon spokesman, said the U.S. would welcome Russia’s direct involvement in battling Islamic State extremists in Syria, but not if done in coordination with Mr. Assad.

“We welcome Russia participating in the global anti-ISIL efforts, but to do that via the Assad regime is unhelpful and potentially destabilizing,” he said.

A principal concern for the U.S. is the risk of accidental clashes or other mishaps in a Syrian airspace where the U.S. and its allies are flying round-the-clock surveillance and bombing missions against Islamic State forces.

Trust between U.S. and Russian defense establishments has broken down in acrimony over the Kremlin’s war in Ukraine, leaving Washington and Moscow without a reliable channel for what military planners call “de-confliction.”

In one clear sign of the breakdown, Secretary of Defense Ash Carter, who took over the Pentagon job in February, hasn’t yet spoken to his Russian counterpart, Sergey Shoygu.

Last week Bulgaria denied Russia overflight rights for Syria transit, and the Russians have been using an alternate air route through Iraqi airspace, officials said.

In their Syria airlift, the Russians have followed established civil-aviation routes, making it harder for the U.S. to exert any over pressure either directly on Moscow or on the overflight countries.

“We’ve been in touch with all of our allies, partners and friends in the area to encourage them to ask hard questions about who and what is flying through their airspace,” Capt. Davis said.

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