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The leader of Lebanon's Hezbollah group vowed Friday to keep fighting in Syria "wherever needed" and said his Shiite Muslim group has made a "calculated" decision to defend the Syrian regime no matter what the consequences.

The comments by Sheik Hassan Nasrallah in a speech to supporters in southern Beirut signaled for the first time the Iranian-backed group will stay involved in the civil war raging next door after helping President Bashar Assad's army recapture a key town in Syria's central Homs province from rebels.

President Barack Obama has authorized lethal aid to Syrian rebels after the U.S. announced it had conclusive evidence that the Syrian regime used chemical weapons. U.S. officials are still grappling with what type and how much weaponry to send, but the announcement buoyed opposition forces, which have found themselves heavily outgunned and outmanned by the Hezbollah-backed regime.

The Syrian government on Friday dismissed U.S. charges that it used chemical weapons as "full of lies," accusing Obama of resorting to fabrications to justify his decision to arm Syrian rebels.

U.S. officials said the administration could provide the rebel fighters with a range of weapons, including small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired rocket-propelled grenades and other missiles. The officials insisted on anonymity in order to discuss internal administration discussions with reporters.

Hezbollah has come under harsh criticism at home and abroad for sending its gunmen to Qusair, and Nasrallah's gamble in Syria primarily stems from his group's vested interest in the Assad regime's survival. The Syrian government has been one of Hezbollah's strongest backers for decades and the militant group fears that if the regime falls it will be replaced by a U.S.-backed government that will be hostile to Hezbollah.

Nasrallah said verbal and other attacks against his militant group "only serve to increase our determination."

"We will be where we should be, we will continue to bear the responsibility we took upon ourselves," Nasrallah said. "There is no need to elaborate...  we leave the details to the requirements of the battlefield."

Assad's forces, aided by fighters from Lebanon's militant group Hezbollah, captured Qusair on June 5, dealing a heavy blow to rebels who had been entrenched in the strategic town for over a year.

Since then, the regime has shifted its attention to recapture other areas in the central Homs province and Aleppo to the north.

A visibly angry Nasrallah did not say outright whether his fighters would go as far as fighting in Aleppo, but his words strongly suggested the group was prepared to fight till the end.

"After Qusair for us will be the same as before Qusair," he said. "The project has not changed and our convictions have not changed."

Nasrallah reiterated that the fight in Syria was one against the "American, Israeli and Takfiri project" that was meant to destroy Syria, which along with Iran has been the group's main backer. Takfiri Islamists refers to an ideology that urges Sunni Muslims to kill anyone they consider an infidel.

Much of the group's arsenal, including tens of thousands of rockets, is believed to have come from Iran via Syria or from Syria itself.

In addition to the increased military aid, the U.S. also announced Thursday it had conclusive evidence that Assad's regime has used chemical weapons, including the nerve agent sarin, on a small scale against opposition forces. The White House said multiple chemical attacks last year killed up to 150 people.

Obama has said the use of chemical weapons cross a "red line," triggering greater U.S involvement in the crisis.

"The White House has issued a statement full of lies about the use of chemical weapons in Syria, based on fabricated information," a statement issued Friday by the Syrian Foreign Ministry said. "The United States is using cheap tactics to justify President Barack Obama's decision to arm the Syrian opposition," it said.

The statement also accused the U.S. of "double standards," saying America claims to combat terrorism while providing support for "terrorist" groups in Syria, such as Jabhat al-Nusra, with arms and money. The group, also known as the Nusra Front, is an al Qaeda affiliate that has emerged as one of the most effective rebel factions in Syria.

The commander of the main Western-backed rebel group fighting in Syria said he hoped that U.S. weapons will be in the hands of rebels in the near future.

"This will surely reflect positively on the rebels' morale, which is high despite attempts by the regime, Hezbollah and Iran to show that their morale after the fall of Qusair deteriorated," Gen. Salim Idris told Al-Arabiya TV.

Loay AlMikdad, a spokesman for the Free Syrian Army, said Idris will begin meeting with international players on Saturday to work out the details of the weapons and their delivery.

"We encourage them to take a decision in this relation, by establishing a no-fly zone either all over Syria or areas they choose based on their technical or military considerations on the ground," he said, adding that would ensure safe areas for civilians. "We hope they start arming immediately. Any delay costs blood of Syrians. It is not water, it is blood of the Syrians, women and children and its future."

AlMikdad said the rebels have asked for shoulder propelled rockets, thermal anti-tank missiles, anti-aircrafts missiles, surface to surface missiles and armored vehicles.

The regime's advances have added urgency to U.S. discussions on whether to provide the rebels with weapons. The United Nations said this week that nearly 93,000 people have been confirmed dead in Syria's civil war, but the actual number is believed to be much higher.

Russia, a staunch ally of Assad, disputed the U.S. charge that Syria used chemical weapons against the rebels.

President Vladimir Putin's foreign affairs adviser, Yuri Ushakov, told reporters that the information provided by U.S. officials to Russia "didn't look convincing."

But he said there was no talk yet about whether  Russia could retaliate to the U.S. move to supply weapons to the Syrian rebels by delivering the S-300 air defense missile systems to the regime.

"We aren't competing over Syria, we are trying to settle the issue in a constructive way," he said.

Ushakov warned that providing such assistance could derail efforts to convene a Syria peace conference. The main opposition coalition has already said it would not attend, all but scuttling the initiative.

Alexey Pushkov, chairman of Russia's parliamentary foreign affairs committee, wrote on his Twitter account Friday that "the data on Assad's use of chemical weapons were faked in the same place as the lie about (Saddam) Hussein's weapons of mass destruction," referring to the deposed Iraqi dictator.

"Obama is going down the route of G. Bush," he added, in reference to former President George W. Bush's assertion -- never proven, but used to justify the invasion of Iraq -- that Saddam Hussein possessed weapons of mass destruction.

A lawmaker in Damascus echoed those comments.

"This reminds us of what America did in the prelude to the invasion of Iraq by releasing fabrications and lies to the international community that Iraq had weapons of mass destruction," said Issam Khalil, a member of Assad's Baath party.

In Friday's violence, Syrian troops and rebels fought some of the heaviest battles in months in Aleppo, Syria's largest city, activists said.

The Britain-based Syrian Observatory for Human Rights said the clashes were concentrated in the city's eastern rebel-held neighborhood of Sakhour, calling the fighting "the most violent in months." It said regime troops attacked the neighborhood from two directions but failed to advance, suffering casualties.