President Obama made the decision after his
administration concluded Syrian forces under Bashar al-Assad were using
chemical weapons, a 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 rebels, the foreign ministry said.
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, who urged Syria to
let the UN "investigate all reports of chemical weapons use".
British Foreign Secretary William Hague said
the UK agreed with Washington's assessment and said an urgent response to the Syria
crisis would be discussed at the G8 this week.
Russia 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.
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 - and 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 in 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.
'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.
Now the West is lining up to try and help the
rebels, but that is likely to take many months with more bloodshed and
destruction, our correspondent adds.
The White House announcement immediately shook
up the ongoing debate in Washington DC over how the US might provide assistance
to the rebels.
Republican Senators John McCain and Lindsey
Graham, who have been particularly strident in their calls for military aid,
said the finding must change US policy in Syria. They called for further
action, saying US credibility was on the line.
"A decision to provide lethal assistance,
especially ammunition and heavy weapons, to opposition forces in Syria is long
overdue, and we hope the president will take this urgently needed step,"
they said in a joint statement.
"But providing arms alone is not
sufficient. The president must rally an international coalition to take
military actions to degrade Assad's ability to use airpower and ballistic
missiles and to move and resupply his forces around the battlefield by
air."
The White House announcement came on the same
day the United Nations said the number of those killed in the Syrian conflict
had risen to more than 93,000 people.
A UN report released on Thursday found at
least 5,000 people have been dying in Syria every month since last July, with
30,000 killed since November.
More than 80% of those killed were men, but
the UN's Office of the High Commissioner for Human Rights (OHCHR) says it has
also documented the deaths of more than 1,700 children under the age of 10.
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