Lebanese troops blocked streets in Beirut with tanks and barbed wire for several hours on Sunday after the killing of a protester outside the Iranian embassy raised factional tensions already inflamed by the war in Syria.

The man died during a clash between rival groups of Shi'ite Muslims after militiamen from the Iranian-backed Hezbollah movement opened fire when protesters drew up at the embassy, the latest sign of Syria's violence spilling over to its neighbours.

In Syria itself, fighting intensified in the north, where rebels said President Bashar al-Assad's forces and their Lebanese Hezbollah allies were preparing an offensive after success last week in seizing a strategic town further south.

In the past week Assad's forces and Hezbollah captured the town of Qusair, which controls vital supply routes across Syria and with Lebanon, a sign of reversing momentum after the rebels seized swathes of the country in the second half of last year.

Battles raged on Sunday near Al-Nubbul and Zahra, two rural Shi'ite Muslim enclaves outside the commercial hub Aleppo in Syria's north, and intensified in Aleppo itself.

"The aim is to use the two villages as forward bases to make advances in Aleppo and its countryside," said Brigadier General Mustafa Al-Sheikh, a rebel commander and former senior officer in Assad's military, referring to government tactics.

"The regime considers that it has received a shot in the arm after the Qusair battle, but they will find that it will not be easy to advance in Aleppo," Sheikh said, speaking from an undisclosed location in northern Syria.

The civil war now pits Assad, from the Alawite offshoot of Shi'ite Islam, and Shi'ite Hezbollah against mainly Sunni Muslim rebel groups. Assad is backed by Shi'ite Iran and armed by Russia. The rebels are armed by Sunni Arab countries Saudi Arabia and Qatar, and backed by Turkey and the West.

Much of the north near the Turkish border has been held by rebels since last year and frontlines inside Aleppo itself have been largely static for months.

An article in the semi-official Syrian al-Watan daily said the Syrian army was "deploying heavily in the countryside near Aleppo in preparation for a battle that will be fought inside the city and on its outskirts."

"Besieged areas will be freed in the first stages and troops which have been on the defensive will go on the offensive," the article said.

Activists said at least ten rebel fighters and six loyalist troops were killed in intensifying combat in the last 24 hours in Aleppo, Syria's largest metropolis, which has been divided into rebel-held and loyalist controlled sectors for a year.

Sheikh said the army has been using helicopters to re-enforce Nubbul and Zahra with loyalist troops including Hezbollah fighters and recruits from Iraq. There was no independent confirmation of any Hezbollah presence near Aleppo.

Hezbollah has pledged to fight alongside Assad until victory in the Syrian war, in which at least 80,000 people have been killed. It does not comment on the specific activities of its fighters in Syria.

Hezbollah Participation

Hezbollah's participation raises the prospect of fighting spreading to Lebanon, which has never fully recovered from its own 1975-1990 civil war.

In Beirut, the Lebanese army, which has limited means to impose itself on armed factions, deployed armoured vehicles and set up roadblocks to cordon the city centre and neighbourhoods controlled by Hezbollah. Traffic was restored toward evening.

Demonstrators from a variety of groups, including Sunni and Shi'ite Muslims and Christians, in protest against Hezbollah's newly prominent role supporting Assad. When protesters from a small Shi'ite party opposed to Hezbollah arrived at the Iranian embassy in a bus, a Reuters journalist saw them clash with black-clad Hezbollah militiamen, who opened fire.

Lebanese security officials said one of the protesters, who was unarmed, was killed and several people were hurt.

"What happened today makes us feel there is a very difficult period ahead. We are bringing disasters upon ourselves by interfering in others' affairs," said hotel owner Ali Hammoud. "No one will come to Lebanon now; our concern now is just to stay alive."

A member of the Lebanese parliament who supports the Sunni former prime minister Saad Hariri said Hezbollah must pull its fighters out. "Hezbollah has driven Lebanon into a tunnel without end," said Nuhad Mashnouq.

Divisions

In Jordan, the United States began military exercises on Sunday that have seen it deploy Patriot anti-aircraft missiles, fighter jets and 4,500 troops to the country, an ally neighbouring Syria.

U.S. officials have said they could leave the new weapons in place after the exercises are over. Russia has complained about the deployment, especially of the Patriots, which it sees as potentially useful if the United States and its allies want to intervene militarily beginning with a no-fly zone.

More than 4,500 American troops, around 3,000 Jordanians, and 500 soldiers from Britain, Saudi Arabia and other countries were taking part in the exercises, less than 75 miles (120 km) from the Syrian border, said military officials.

"The drills having nothing to do with any objective related to what is happening in Syria," the top army commander in charge of Jordanian troops, Major General Awni al-Adwan, told reporters during the launch of the exercises on Sunday.

U.S. Major General Robert G. Catalanotti told a joint news conference the Eager Lion events would increase "our ability to operate together in any upcoming contingency".

The exercises also involve drills on handling chemical strikes, which Syria's government and rebels have accused each other of carrying out.

The United States and Russia have been trying to bridge differences by pushing the warring sides to peace talks in Geneva, but divisions among the opposition and wrangling among the major powers over whether Assad must step down have left the prospects for the conference unclear.

Britain and France have broken ranks with other European powers and the United States to say they may join Saudi Arabia and Qatar in arming the rebels as a way to end a war that has killed over 80,000 people. British Foreign Secretary William Hague said on Sunday London was still "very reluctant" to do so.

"People have understandable concerns about the idea of sending arms to anybody in Syria," he said. "On the other hand, at the moment, people are being killed in huge numbers while the world denies them the right to defend themselves."

Israel, which has bombed what it suggests were Iranian missiles in Syria headed for Hezbollah, said on Sunday it aimed to stay out of the war, though it is concerned by fighting on the U.N.-policed ceasefire line in the Golan Heights that might bring Islamists toward its border.

"Israel is not getting involved in the civil war in Syria, as long as the fire is not directed at us," Prime Minister Benjamin Netanyahu told his cabinet in broadcast remarks.