ST PETERSBURG, Russia/UNITED NATIONS (Reuters) - Russian President Vladimir Putin cautioned the West on Friday against arming Syrian rebel forces, which he said included "terrorist" groups, and warned that a swift exit by President Bashar al-Assad risked creating a dangerous power vacuum.
"If the United States ... recognizes one of the key Syrian opposition organizations, al-Nusra, as terrorist ... how can one deliver arms to those opposition members?" Putin told a panel with German Chancellor Angela Merkel. "Where will they end up? What role will they play?"
Putin defended his own country's arm sales to the embattled Syrian government as "entirely legal, repeating Russia's position that outsiders should not determine the fate of Assad and Syria.
"If Assad goes today, a political vacuum emerges - who will fill it?" Putin said at a later news conference with Merkel. "Maybe those terrorist organizations. Nobody wants this - but how can it be avoided? After all, they are armed and aggressive."
The only solution, he said, was an international peace conference that Russia and the United States are seeking to convene.
A U.N. human rights investigator warned on Friday that an increased flow of arms to Syria's government and rebel forces would likely result in increased war crimes in a two-year civil war that has killed some 93,000 people.
"States who provide arms have responsibilities in terms of the eventual use of those arms to commit ... war crimes or crimes against humanity," said Paulo Pinheiro, chairman of a U.N. commission of inquiry on rights violations in Syria.
With Russia and Iran arming Assad's forces, and Lebanon's Hezbollah fighters joining the war on his behalf, Western powers have agreed to step up aid to the mainly Sunni rebels.
U.S. President Barack Obama, citing the Syrian government's alleged use of chemical weapons, decided last week to provide military aid to rebels fighting to overthrow Assad.
Obama said on Friday the United States was leaving about 700 combat-equipped troops in Jordan after a training exercise there, at the request of Amman. The United States previously decided to leave Patriot missiles and warplanes in the country.
Jordan fears a spillover of the Syrian war into its territory, where an estimated half million Syrian refugees have fled to escape the bloodshed.
With both countries keeping an eye on the war, Obama told Congress in a letter that the troops would remain until the security situation became such that they were no longer needed.
The European Union lifted its arms embargo on Syria last month. Britain and France have spoken in favor of potentially arming the rebels, but have not yet taken any decisions.
Western powers had been reluctant in the past to arm the rebels because of concerns about the rising strength of Sunni Islamist insurgents who have pledged loyalty to al Qaeda.
France sent 16 tonnes (1 tonne = 1.102 tons) of medical aid to northern Syria on Friday, including antidotes for nerve agents, as rebels prepared to face an assault on the city of Aleppo by Assad's forces.
GROWING MIDDLE EAST DIVIDE
A U.N. spokeswoman said senior U.S. and Russian officials would meet with the international mediator on Syria in Geneva on Tuesday to discuss a peace conference.
An international peace conference is not expected to occur before August after G8 leaders clashed with Russia over the nature of a transitional government.
Foreign ministers of the Friends of Syria group of nations, which backs the opposition, will meet in Qatar on Saturday to discuss how to help the rebel Free Syrian Army defend the northern city of Aleppo.
The insurgents have suffered a recent series of battlefield setbacks and are besieged on the outskirts of Damascus by advancing government forces, who have begun to regain the upper hand.
Two years of fighting have dragged Syria's neighbors into a deadly confrontation between Shi'ite Iran supporting Assad, whose Alawite sect derives from Shi'ite Islam, and Sunni Arab Gulf nations backing the rebels.
In Beirut, the Lebanese army sealed off the parliamentary district on Friday and threatened stern action against violence after a night of unrest triggered by the Syrian war and political paralysis at home. The fighting in Syria has driven half a million Syrian refugees into Lebanon.
Sectarian violence has intensified in Lebanon because of the Syrian conflict across the border, where Lebanon's Shi'ite militia Hezbollah and Lebanese Sunni gunmen have joined opposing sides of the war.
A Cabinet minister in Iraq's Shi'ite-led government said on Friday that thousands of Shi'ite Muslims from Iraq and beyond would take up arms against Sunni al Qaeda 'savages" if fellow Shi'ites or their shrines came under further attack.
Hadi al-Amiri, Iraq's transport minister, told Reuters it would be impossible to "sit idle while the Shi'ites are being attacked," while the United States and Western allies arm and finance the mainly Sunni rebels.
Spanish police arrested eight people on Friday in Spain's North African enclave of Ceuta on suspicion of recruiting fighters for a branch of al Qaeda in Syria.
(Additional reporting by John Irish in Paris, Gabriela Baczynska in Moscow, Samia Nakhoul and Suadad al-Salhy in Baghdad, Dominic Evans in Beirut, Stephanie Nebehay in Geneva; Paul Day in Madrid and Steve Holland in Washington; Writing by Peter Cooney; Editing by Eric Walsh)