Saudi Arabia has financed a large purchase of infantry weapons from Croatia and quietly funneled them to antigovernment fighters in Syria in a drive to break the bloody stalemate that has allowed President Bashar al-Assad to cling to power, according to American and Western officials familiar with the purchases.

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.