* Air strike levels several houses, dozens search rubble
* Saudi Arabia tells citizens to leave Lebanon after kidnap
* UN report: Assad's forces and rebels have committed war crimes
* UN investigators say state crimes outweigh those of rebels
* Islamic grouping set to suspend Syria at Mecca summit
By Hadeel Al Shalchi
AZAZ, Syria, Aug 15 (Reuters) - A Syrian air strike killed 30 people in a rebel-held town on Wednesday, a local doctor said, and a mass kidnapping linked to Syria in neighbouring Lebanon raised the prospect of sectarian violence spreading.
That citizens of Turkey and Saudi Arabia, key supporters of the Sunni Muslim insurgency, were among those seized by Lebanese Shi'ites prompted Gulf states to urge citizens to leave Lebanon. It also underscored how the Syrian conflict is dividing the region along sectarian lines as world powers remain deadlocked.
Doctor Mohammad Lakhini said at a hospital in Azaz, in the north near the Turkish border, that scores of people there were wounded in the raid by President Bashar al-Assad's air force. It reduced several houses to rubble and dozens of men clawed through the concrete and metal debris looking for survivors.
In video posted by activists earlier on Wednesday, residents in Azaz - close to the major urban battleground of Aleppo - screamed and shouted "God is greatest" as they carried bloodied bodies from collapsed concrete buildings.
The opposition Syrian Observatory for Human Rights said dozens had been killed. One activist in the town said at least 30 bodies had been found and rescuers were searching for more.
The video footage, which could not be immediately verified, showed crowds of residents wrestling with steel bars and pulling away a giant slab of concrete to reveal the dust-covered arm of a child. "This is a real catastrophe," said an activist who gave his name only as Anwar. "An entire street was destroyed."
Seven Lebanese hostages being held in Azaz were also wounded, with four others still missing, a rebel commander said.
"The building they were in was hit," rebel commander Ahmed Ghazali told the Lebanese news channel Al Jadeed.
"We were able to remove seven from the wreckage. They are wounded, and some of the injuries are serious."
Assad's forces have increasingly used helicopter gunships and warplanes against the lightly-armed insurgents - elements in fresh accusations of war crimes leveled by United Nations human rights investigators on Wednesday.
The Syrian civil war has taken on overtly sectarian overtones, with most rebels belonging to the Sunni Muslim majority, fighting against government forces rooted in Assad's Alawite minority, an offshoot of Shi'ite Islam.
Regional powers are being drawn into the fight, with Sunni-led Saudi Arabia, Qatar and Turkey supporting the rebels and Shi'ite Iran backing Assad's government. Fighting between Sunnis and Shi'ites lay behind long civil wars in Syria's neighbours Iraq and Lebanon, and the West fears the violence could spread.
In Lebanon, gunmen belonging to a powerful Shi'ite clan abducted more than 20 men, including at least one Turk, one Saudi and several Syrian anti-Assad fighters, in retaliation for the capture of one of their kinsmen by rebels in Damascus.
The incident, in an area of Lebanon controlled by Hezbollah Shi'ite militants long allied to Assad and supported by Iran, raised the prospect of Syria's sectarian violence spilling over to its neighbour. Mass kidnapping was a perennial tactic in Lebanon's own sectarian civil war from 1975-1990.
Members of the Meqdad clan said they had carried out the kidnappings in retaliation for the capture of kinsman Hassan al-Meqdad by anti-Assad rebels in Damascus two days earlier.
They threatened to carry out more abductions of Qataris, Turks and Saudis. Saudi Arabia and the United Arab Emirates told their citizens to leave Lebanon - potentially dealing a blow to Beirut's reviving tourist business.
Syria's civil war has polarised Lebanon, with Shi'ites rallying behind Assad and Sunnis backing his enemies.
In Damascus, a bomb exploded in the car park of a hotel used by U.N. monitors, but several military buildings are also in the vicinity and it was not clear what the target was. No U.N. staff were hurt in the blast which set a fuel tanker ablaze.
State media said three people were wounded in the bombing and several rebels were killed or captured in a separate gunbattle with security forces in the western district of Mezze.
U.N. emergency relief coordinator Valerie Amos, on a mission to seek more access for aid deliveries, was meeting European Union officials in Damascus when the bomb went off.
She herself was unable to reach the town of Douma, a trouble spot just north of the capital, due to bombardment.
"Waiting at checkpoint to get into Duma. Sounds of shelling. Could not enter," Amos tweeted. The authorities told her she had been turned back for her own safety, her spokesman said later.
As the violence intensified, U.N. human rights investigators accused forces loyal to Assad of committing war crimes and crimes against humanity.
They said rebels had also committed war crimes, but the violations "did not reach the gravity, frequency and scale" of those by state forces and the pro-Assad shabbiha militia.
"The commission found reasonable grounds to believe that government forces and the shabbiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law, including unlawful killing, torture, arbitrary arrest and detention, sexual violence, indiscriminate attack, pillaging and destruction of property," said the 102-page report by the independent investigators led by Paulo Pinheiro.
Last month, Assad's troops successfully counter-attacked after rebels seized parts of Damascus. They are still trying to dislodge insurgents from Aleppo, Syria's biggest city.
A Syrian air strike has wrecked a hospital in a rebel-held area of Aleppo, a doctor there said on Wednesday, an attack that New York-based Human Rights Watch said violated international law. At least two holes gaped in the walls of Al Shifaa Hospital and four floors were heavily damaged by Tuesday's raid.
"If we had lingered just another five minutes, we would have died," said the surgeon, who gave his name only as Younes.
Dust covered hospital beds, incubators were broken and the floor was strewn with rubble. Water from a broken tank had leaked out, mixing with patches of blood.
Opposition sources say 18,000 people have been killed since the uprising against Assad erupted in March last year. The bloodshed has divided regional and world powers, making peace efforts fruitless and paralysing the U.N. Security Council.
Most Western and Arab governments have called on Assad to go, saying his government's violent response to initially peaceful protests give him no place in a future Syria.
Russia has opposed tougher U.N. sanctions against Damascus, a long-time strategic ally, but denies it is actively helping Assad remain in power. Foreign Minister Sergei Lavrov accused Western governments of reneging on a deal among world powers made on June 30 to push for a transitional government in Syria.
Washington shot back that it was Russia and China which had blocked efforts to pass a U.N. Security Council resolution on a transition. Moscow has said the Western powers should not make the removal of Assad a pre-condition for such a handover.
Muslim heads of state were expected to suspend Syria from the Organisation of Islamic Cooperation at a summit in Mecca on Wednesday, over the objections of Iran, Assad's closest ally.
The 57-member body's rebuke is mostly symbolic, but it shows Syria's isolation - as well as Iran's - across much of the Sunni-majority Islamic world.