By Mark Hosenball, Phil Stewart and Matt Spetalnick

WASHINGTON (Reuters) - Senior U.S. lawmakers have begun probing possible intelligence lapses over Moscow’s intervention in Syria, concerned that American spy agencies were slow to grasp the scope and intention of Russia’s dramatic military offensive there, U.S. congressional sources and other officials told Reuters.

A week after Russia plunged directly into Syria’s civil war by launching a campaign of air strikes, the intelligence committees of the U.S. Senate and House of Representatives want to examine the extent to which the spy community overlooked or misjudged critical warning signs, the sources said.

Findings of major blind spots would mark the latest of several U.S. intelligence misses in recent years, including Moscow’s surprise takeover of Ukraine’s Crimea region last year and China’s rapid expansion of island-building activities in the South China Sea.

Though spy agencies have sought to ramp up intelligence gathering on Russia since the crisis over Ukraine, they continue to struggle with inadequate resources because of the emphasis on counter-terrorism in the Middle East and the Afghanistan-Pakistan region, according to current and former U.S. officials.

A senior administration official, who also asked not to be identified, insisted that there were “no surprises” and that policymakers were “comfortable” with the intelligence they received in the lead-up to the Russian offensive.

Spy agencies had carefully tracked Russian President Vladimir Putin’s build-up of military assets and personnel in Syria in recent weeks, prompting White House criticism and demands for Moscow to explain itself.

But intelligence officers – and the U.S. administration they serve - were caught mostly off-guard by the speed and aggressiveness of Putin’s use of air power as well as a Russian target list that included U.S.-backed rebels, according to the officials, who spoke on condition of anonymity.

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"They saw some of this going on but didn't appreciate the magnitude," one of the sources told Reuters.

Russia’s sudden move to ramp up its military involvement in the Syria crisis has thrown Obama's Middle East strategy into doubt and laid bare an erosion of U.S. influence in the region.

A shortage of reliable information and analysis could further hamper President Barack Obama’s efforts to craft a response on Syria to regain the initiative from Washington’s former Cold War foe.

BEHIND THE CURVE?

It is unclear how his administration could have reacted differently with better intelligence, though advance word of Putin’s attack plans might have allowed U.S. officials to warn the moderate Syrian opposition that they could end up in Russia's line of fire.

Obama, who is reluctant to see America drawn deeper into another Middle East conflict, has shown no desire to directly confront Russia over its Syria offensive – something Moscow may have taken as a green light to escalate its operations.

Syrian troops and militia backed by Russian warplanes mounted what appeared to be their first major coordinated assault on Syrian insurgents on Wednesday and Moscow said its warships fired a barrage of missiles at them from the Caspian Sea, a sign of its new military reach.

Russia's military build-up now includes a growing naval presence, long-range rockets and a battalion of troops backed by Moscow's most modern tanks, the U.S. ambassador to NATO said.

The U.S. administration believes it now has a better understanding at least of Putin’s main motive – to do whatever it takes to prop up Syrian President Bashar al-Assad. But Washington remains uncertain exactly how much further Putin is willing to go in terms of deployment of advanced military assets, the U.S. officials said.

The lack of clarity stems in part from the limited ability of U.S. intelligence agencies to discern what Putin and a tightly knit circle of advisers are thinking and planning.

In a tense meeting with Putin at the United Nations early last week, Obama was not given any advance notice of Russia’s attack plans, aides said. Russian air strikes began two days later, including the targeting of CIA-trained “moderate” anti-Assad rebels, though Moscow insisted it only hit Islamic State insurgents.

“They did not expect the speed with which Putin ramped things up," said Michael McFaul, Obama’s former ambassador to Moscow. "He likes the element of surprise."

U.S. intelligence agencies did closely follow and report to policymakers Russian moves to sharply expand infrastructure at its key air base in Latakia as well as the deployment of heavy equipment, including combat aircraft, to Syria, officials said.

“We’re not mind readers,” the senior administration official said. “We didn’t know when Russia would fly the first sortie, but our analysis of the capabilities that were there was that they were there for a reason.”

However, several other officials said U.S. agencies were behind the curve in assessing how far the Russians intended to go and how quickly they intended to launch operations.

In fact, right up until a White House briefing given shortly after the bombing began, Obama press secretary Josh Earnest declined to draw "firm conclusions" on Russia's strategy.

CONFUSION OVER RUSSIAN INTENT

One source suggested that U.S. experts initially thought the Russian build-up might have been more for a military "snap exercise" or a temporary show of force than preparations for sustained, large-scale attacks on Assad's enemies.

Another official said that after initial review, congressional oversight investigators believe that "information on this was not moving quickly enough through channels” to policymakers.

And another source said there had been a "lag of a week" before agencies began voicing full-throated alarm about imminent Russian military operations.

The senior administration official said, however, that “I don’t think anybody here perceived a gap” in intelligence.

In their reviews of how U.S. intelligence handled the Syria build-up, officials said congressional intelligence committees would examine reports issued by the agencies and question officers involved in the process, according to congressional and national security sources. At the moment, no public hearings are planned, the officials said.

Though the senior administration official denied the intelligence community was paying any less attention to Syria, John Herbst, a former U.S. ambassador to Ukraine, said that not enough intelligence assets had been devoted to analyzing Putin’s “aggressive policies”.

McFaul, who took the view that the Obama administration had been largely on top of the situation as Putin prepared his offensive, said that a faster or more precise intelligence assessment would probably have done little to change the outcome.

“What difference would it make if we had known 48 hours ahead of time?” asked McFaul, who now teaches at Stanford University in California. “There still wouldn’t have been any better options for deterring Putin in Syria.”

(Additional reporting by Lesley Wroughton and Roberta Rampton, Writing by Matt Spetalnick; editing by Stuart Grudgings)