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A recoilless gun that originated in the former Yugoslavia.

The weapons began reaching rebels in December via shipments shuttled through Jordan, officials said, and have been a factor in the rebels' small tactical gains this winter against the army and militias loyal to Mr. Assad.

The arms transfers appeared to signal a shift among several governments to a more activist approach to assisting Syria's armed opposition, in part as an effort to counter shipments of weapons from Iran to Mr. Assad's forces. The weapons' distribution has been principally to armed groups viewed as nationalist and secular, and appears to have been intended to bypass the jihadist groups whose roles in the war have alarmed Western and regional powers.

For months regional and Western capitals have held back on arming the rebels, in part out of fear that the weapons would fall into the hands of terrorists. But officials said the decision to send in more weapons is aimed at another fear in the West about the role of jihadist groups in the opposition. Such groups have been seen as better equipped than many nationalist fighters and potentially more influential.

The action also signals the recognition among the rebels' Arab and Western backers that the opposition's success in pushing Mr. Assad's military from much of Syria's northern countryside by the middle of last year gave way to a slow, grinding campaign in which the opposition remains outgunned and the human costs continue to climb.

Washington's role in the shipments, if any, is not clear. Officials in Europe and the United States, including those at the Central Intelligence Agency, cited the sensitivity of the shipments and declined to comment publicly.

But one senior American official described the shipments as "a maturing of the opposition's logistical pipeline." The official noted that the opposition remains fragmented and operationally incoherent, and added that the recent Saudi purchase was "not in and of itself a tipping point."

"I remain convinced we are not near that tipping point," the official said.

The official added that Iran, with its shipments to Syria's government, still outstrips what Arab states have sent to the rebels.

The Iranian arms transfers have fueled worries among Sunni Arab states about losing a step to Tehran in what has become a regional contest for primacy in Syria between Sunni Arabs and the Iran-backed Assad government and Hezbollah of Lebanon.

Another American official said Iran has been making flights with weapons into Syria that are so routine that he referred to them as "a milk run." Several of the flights were by an Iranian Air Force Boeing jet using the name Maharaj Airlines, he said.

While Persian Gulf Arab nations have been sending military equipment and other assistance to the rebels for more than a year, the difference in the recent shipments has been partly of scale. Officials said multiple planeloads of weapons have left Croatia since December, when many Yugoslav weapons, previously unseen in the Syrian civil war, began to appear in videos posted by rebels on YouTube.

Many of the weapons — which include a particular type of Yugoslav-made recoilless gun, as well as assault rifles, grenade launchers, machine guns, mortars and shoulder-fired rockets for use against tanks and other armored vehicles — have been extensively documented by one blogger, Eliot Higgins, who writes under the name Brown Moses and has mapped the new weapons' spread through the conflict.

He first noticed the Yugoslav weapons in early January in clashes in the Dara'a region near Jordan, but by February he was seeing them in videos posted by rebels fighting in the Hama, Idlib and Aleppo regions.

Officials familiar with the transfers said the arms were part of an undeclared surplus in Croatia remaining from the 1990s Balkan wars. One Western official said the shipments included "thousands of rifles and hundreds of machine guns" and an unknown quantity of ammunition.

Croatia's Foreign Ministry and arms-export agency denied that such shipments had occurred. Saudi officials have declined requests for interviews about the shipments for two weeks. Jordanian officials also declined to comment.

Danijela Barisic, a spokeswoman for Croatia's Ministry of Foreign Affairs, said that since the Arab Spring began, Croatia had not sold any weapons to either Saudi Arabia or the Syrian rebels. "We did not supply arms," she said by telephone.

Igor Tabak, a Croatian military analyst, said that after a period when many countries in the former Yugoslavia sold weapons from the Balkan wars on black markets, Croatia, poised this year to join the European Union, now strictly adheres to international rules on arms transfers.

"I can't imagine bigger quantities of weapons being moved without state sanctioning," he said. "It is not impossible, but it is just very improbable." He added that it was possible that such weapons could be moved by the intelligence services, though he offered no evidence that that was the case.

Syria's rebels have acquired their arms through a variety of means, including smuggling from neighboring states, battlefield capture, purchases from corrupt Syrian officers and officials, sponsorship from Arab governments and businessmen, and local manufacture of crude rockets and bombs. But they have remained lightly equipped compared with the government's conventional military, and have been prone to shortages.

An official in Washington said the possibility of the transfers from the Balkans was broached last summer, when a senior Croatian official visited Washington and suggested to American officials that Croatia had many weapons available should anyone be interested in moving them to Syria's rebels.

At the time, the rebels were advancing slowly in parts of the country, but were struggling to maintain momentum amid weapons and ammunition shortages.

Washington was not interested then, the official said, though at the same time, there were already signs of limited Arab and other foreign military assistance.

Both Ukrainian-made rifle cartridges that been purchased by Saudi Arabia and Swiss-made hand grenades that had been provided to the United Arab Emirates were found by journalists to be in rebel possession.

And Belgian-made rifles of a type not known to have been purchased by Syria's military have been repeatedly seen in rebel hands, suggesting that one of Belgium's previous rifle customers had transferred the popular weapons to the rebels.

But several officials said there had not been such a visible influx of new weapons as there has been in recent weeks.

By December, as refugees were streaming over Syria's borders into Turkey and Jordan amid mounting signs of a wintertime humanitarian crisis, the Croatian-held weapons were back in play, an official familiar with the transfers said.

One Western official familiar with the transfers said that participants are hesitant to discuss the transfers because Saudi Arabia, which the official said has financed the purchases, has insisted on secrecy.

Jutarnji list, a Croatian daily newspaper, reported Saturday that in recent months there had been an unusually high number of sightings of Jordanian cargo planes at Pleso Airport in Zagreb, Croatia's capital.

The newspaper said the United States, Croatia's main political and military ally, was possibly the intermediary, and mentioned four sightings at Pleso Airport of Ilyushin 76 aircraft owned by Jordan International Air Cargo. It said such aircraft had been seen on Dec. 14 and 23, Jan. 6 and Feb. 18. Ivica Nekic, director of the agency in charge of arms exports in Croatia, dismissed the Croatian report as speculation.