The leader of the Syrian opposition made a powerful plea to a reluctant US administration Wednesday for anti-aircraft weapons to end "the nightmare" of civilians under seige from daily barrel bombings.

At the start of his first official visit to Washington, Ahmad Jarba told a US think-tank that opposition forces need "efficient weapons to face these attacks including air raids, so we can change the balance of power on the ground."

This would "open the door for a real political solution," he insisted, committing his Syrian National Coalition to ensuring that any weapons such as anti-aircraft missiles would not fall into the wrong hands.

Equipping the rebels with heavy weapons would both send a powerful signal to the Syrian regime and boost the rebels standing among an increasingly desperate population caught in the crossfire, opposition officials say privately.

Jarba also denounced as a "farce" plans by Syrian President Bashar al-Assad to stand for re-election in polls set for next month, saying it would give him "a licence ... to kill his own people for many years to come."

"He wants to run for office on the dead bodies of the Syrians," Jarba told the US Institute of Peace on the eve of talks with US Secretary of State John Kerry as well as other senior White House and Pentagon officials.

In a bid to show a united opposition front, he is accompanied by the new chief of staff of the Free Syrian Army (FSA), Brigadier General Abdelilah al-Bashir who took over at the head of the military arm after a bitter power tussle with former head General Selim Idriss.

In the past three years some 10 million people had been displaced from their homes in Syria, many of them fleeing abroad, while some 200,000 people had been killed, Jarba maintained.

- 'Don't send your sons to die in Syria' -

But he denied that Assad was gaining the upper hand in the grinding conflict, saying that in towns like Idlib and Aleppo as well as to the south on the coast, the opposition was "moving forward."

But Jarba was speaking on the day that the rebels began to evacuate what had been their stronghold of Homs, under an unprecedented deal that hands the city and its shelled-out ruins back to the regime.

Jarba admitted Homs was "a very important symbol for the Syrian revolution" in which the "inhabitants gave everything."

But its loss was "not the end of the world," he said, adding that what was important was to save lives and "in all battles you have a rhythm" and the rebels would win the city back.

Jarba also stressed Syrians were not calling on the United States or the West "to send their sons to Syria" to fight as in past wars in Iraq or Afghanistan.

"We do not want Americans to die in Syria," he insisted.

But "we do have a problem with the air forces, the air raids and the barrel bombs. This is making our lives a nightmare.

"We do need effective and efficient weapons... and we commit to keep them in the right hands."

Washington is the largest donor of humanitarian aid to the Syrian people, having provided some $1.7 billion in assistance for the refugees and those besieged in the country.

And while it has provided some non-lethal military support such as night-vision goggles, medical supplies, and communications equipment, US President Barack Obama has steered clear of providing heavy weaponry, fearful it could end up in the hands of Islamist groups who have flooded into the country amid the chaos.

Jarba however said, before arriving in the US, that the rebels had received American TOW anti-tank missiles.

A senior US administration official did acknowledge this week that there was "an asymmetry militarily" between the opposition rebels and the Assad regime, and said Washington was looking at ways to change the balance of power on the ground.

US officials insist there can however be no military solution to the three-year war, and argue that what is needed is to create the right environment to resume peace negotiations, which collapsed in failure in February.