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 US had warned any use of chemical weapons would cross a “red line”.
The BBC’s Jim Muir in Beirut says the US announcement is one that the Syrian opposition has been pushing and praying for for months.
It seems clear that President Obama has finally been persuaded, as Britain and France have argued, that the battlefield cannot be allowed to tilt strongly in the regime’s favour, as is currently happening, he adds.
Washington’s “clear” statement was welcomed by Nato Secretary General Anders Fogh Rasmussen.
“Urgent that Syria regime should let UN investigate all reports of chemical weapons use,” he said on his official Twitter feed.
But a spokesperson for UN Secretary General Ban Ki-moon told the BBC that he remained against “any further militarisation” of the conflict in Syria, saying the people there need peace not more weapons.
‘High confidence’
Mr Rhodes, a deputy national security adviser to Mr Obama, said the US intelligence community believed the “Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times over the last year”.
He said intelligence officials had a “high confidence” in their assessment, and also estimated that 100 to 150 people had died from chemical weapons attacks, “however, casualty data is likely incomplete”.
“We have consistently said the use of chemical weapons violates international norms and crosses red lines that have existed in the international community for decades,” Mr Rhodes said.
Mr Rhodes said President Obama had made the decision to increase assistance, including “military support”, to the Supreme Military Council (SMC) and Syrian Opposition Coalition.
He did not give details of the aid, but administration officials have been quoted by US media as saying it will most likely include sending small arms and ammunition.
The New York Times quoted US officials as saying that Washington could provide anti-tank weapons.
Syria’s rebels have been calling for both anti-tank and anti-aircraft weaponry.
The Wall Street Journal reported that Washington is also considering a no-fly zone inside Syria, possibly near the border with Jordan, that would protect refugees and rebels who are training there.
When asked whether Mr Obama would back a no-fly zone over Syria, Mr Rhodes said one would not make a “huge difference” on the ground – but would be costly.
He said further actions would be taken “on our own timeline.”
The CIA is expected to co-ordinate delivery of the military equipment and train the rebel soldiers on how to use it.
Until now, the US has limited its help to rebel forces by providing rations and medical supplies.
Mr Rhodes said the White House hoped the increased support would bolster the effectiveness and legitimacy of both the political and military arms of Syria’s rebels, and said the US was “comfortable” working with SMC chief Gen Salim Idris.
“It’s been important to work through them while aiming to isolate some of the more extremist elements of the opposition, such as al-Nusra,” he said.
A senior pro-Kremlin politician in Russia – an ally of Syria – said US claims of the Assad government’s use of chemical weapons were “fabricated”.
Likening it to when the US wrongly claimed Saddam Hussein held chemical weapons in Iraq, Alexei Pushkov, head of lower house of parliament foreign affairs committee, tweeted: “Obama is taking the same path as George Bush.”
‘Long overdue’
The US decision marks a significant escalation of the proxy war that has been gathering pace in Syria, our Beirut correspondent says.
The support of the West’s regional allies, Qatar and Saudi Arabia, had helped the rebels in the days after the uprising became militarised.
But the tide turned after the Assad government turned to Moscow and Tehran for help. Hezbollah fighters have also been involved in the government’s counter-offensive.
Read more at bbc.com
Obama approves arms for Syria rebels
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The US is to supply direct military aid to the Syrian opposition for the first time, the White House has announced. President Obama made the decision after his administration concluded Syrian forces under Bashar al-Assad were using chemical weapons, a spokesman said.