President Barak Obama made the decision after his administration prepared a report, accusing the Syrian army of using chemical weapons, a White House spokesman said.
Ben Rhodes did not give details about the military aid other than to say it would be "different in scope and scale to what we have provided before".
The Syrian government said the US statement was full of lies.
The White House relied on fabricated information about chemical weapons use in Syria in order to justify its decision to arm the militants, the Syrian foreign ministry said.
The US announcement is one that the Syrian opposition has been pushing for, after they rejected international calls for coming forward to talk and solve the problem which has already taken tens of thousands of lives.
It seems clear that Obama has been persuaded, as Britain and France have argued, that the battlefield cannot be allowed to tilt in favor of the country’s army and government, as is currently happening.
Washington decision comes as in the past months Syrian army has finally gathered its strength to fight against a massive terrorist war to protect people and free some of its border areas from terrorists occupation.
US accusations of use of chemical weapons by Syrian army are doubted by many as the Syrian government has already submitted evidences of several use of the deadly sarin gas by US-backed militants against people in Syria.
Turkish government also announced that they have found sarin in possession of anti-Syria armed groups who stayed in Turkey.
Russia also said it had seen Washington's evidence of chemical weapons use in Syria; "What was presented to us by the Americans does not look convincing," said Yuri Ushakov, President Putin's senior foreign policy adviser.
The sad scene of daily killing is going on in Syria with the country’s army trying hard to save the country from foreign-backed terrorists and the government announcing readiness for engaging in international talks, with no pre-conditions to end the crisis.
According to BBC, a spokesperson for UN Secretary General Ban Ki-moon said he remained against "any further militarization" of the conflict in Syria, saying the people there need peace not more weapons.