BEIRUT -- Al-Qaida-linked gunmen killed a rebel commander in northwestern Syria, an activist group and an opposition spokeswoman said Friday, in a sign of increased tensions and infighting among groups battling the Damascus regime.
The Britain-based Syrian Observatory for Human Rights said members of the Islamic State of Iraq and the Levant -- a reportedly merged group made up of al-Qaida's branches in Iraq and Syria -- were behind the shooting of the Free Syrian Army commander, Kamal Hamami.
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The incident came as Syria's main opposition bloc complained that "elements in the U.S. Congress" are obstructing the Obama administration's efforts to step up support for the rebels fighting to topple President Bashar Assad's regime from power.
The Observatory said Hamami was shot dead late Thursday after militants tried to remove a checkpoint he set up in the Jabal al-Akrad mountain in the coastal province of Latakia. It said two of his men were seriously wounded in the shooting.
Activists have in the past reported occasional clashes between rebel groups and Islamic militants active in rebel-held areas, especially in the north where the opposition has control of a large part of the territory.
There has also been infighting between Kurdish and Arab groups over control of territory captured from government along the border with Turkey in the past year. That fighting subsided after a cease-fire agreement early this year.
Hamami's killing would underline a deepening power struggle between moderate and extremist groups fighting in Syrian civil war.
A spokeswoman for the Syrian National Coalition, Sarah Karkour, said the Free Syrian Army has confirmed Hamami's death at the "hands of the Islamic State in Iraq and Levant." She did not elaborate.
The SNC is a political wing of the Free Syrian Army, or FSA, the umbrella rebel group.
The FSA regrouped in December under a unified rebel command called the Supreme Military Council, following promises of more military assistance once a central council was in place. The Western-backed council is headed by Gen. Salim Idriss, who defected from the Syrian army, and a 30-member group of senior officers. Idriss spent 35 years in the Syrian military and is seen as a secular-minded moderate.
Some FSA units still operate autonomously, however, often fighting alongside more effective groups on the battlefield, including the al-Qaida-affiliated Jabhat al-Nusra, which has led most successful battles for army bases, villages and towns in the north along the border with Turkey.
The group, known in English as The Nusra Front, has claimed responsibility for several car bombs and suicide attacks on military installations and government buildings, including in the capital Damascus, the seat of Assad's power.
The Nusra Front's success on the battlefield has been a contentious issue within al-Qaida.
In April, the head of Iraq's al-Qaida arm announced a merger with the Syrian branch -- a claim the Nusra Front's leader quickly rebuffed. Earlier this month, al-Qaida's global leader, Ayman al-Zawahiri, was quoted as trying to end the squabbling, insisting that the merger should be dissolved.
In June, The Nusra Front claimed responsibility for multiple suicide attacks on security compounds in Damascus that killed at least five people. The group had been silent about its activities for two months and the claim could be an indication it has resumed operations on the battlefield autonomously.
In a statement late Thursday, the Western-backed SNC urged Congress to back arms deliveries to the rebels.
President Barack Obama recently said the U.S. is willing to send weapons to the opposition. Even so, Washington has been reluctant to arm the rebels battling Assad's troops because radical Islamic groups, including some with al-Qaida links, have emerged as their most effective fighting force. Western countries have also been concerned over the lack of unified command among rebel groups.
"The Syrian Coalition is deeply concerned by reports indicating that elements in the U.S. Congress are delaying the administration's efforts to increase its support to the Free Syrian Army" said the statement late Thursday.
The coalition will ensure "that arms will not reach extremist elements," it added.
More than 93,000 people have been killed in the Syrian conflict that erupted in March 2011 with largely peaceful protests against Assad's rule but escalated into a civil war in response to a brutal government crackdown.
Over the past year, the conflict became increasingly sectarian, with mostly-Sunni rebels assisted by foreign fighters while Assad's forces are bolstered by fighters of the Lebanese Shiite group Hezbollah.
The regime in Damascus is also backed by Russia and Iran, and Moscow has continued to supply Assad with weapons throughout the crisis, saying it is fulfilling existing contracts.
In May, Russia acknowledged it has agreed to sell Syria advanced S-300 air defense missiles, which are considered to be the cutting edge in aircraft interception technology. It was not clear if any of those missiles have been shipped to Damascus and Moscow did not give a time frame on the deliveries.
The U.S., its European and Gulf allies have backed the opposition in the conflict, sending funds and nonlethal aid to the rebels.
There have been fissures among Western nations over sending arms to the opposition despite the countries' shared belief that Assad must leave power. Britain and France in May and successfully pushed for the European Union to allow an arms embargo preventing the flow of weapons into Syria to expire. However, both counties emphasized they have no immideate plans to send rebels weapons.