* Cluster munitions banned by most countries
* U.N. envoy Brahimi due in Iran for talks -Iran media
* Syria bans flights by Turkish aircraft over its air space
By Angus MacSwan and Khaled Yacoub Oweis
BEIRUT/AMMAN, Oct 14 (Reuters) - Syrian government forces have dropped Russian-made cluster bombs over civilian areas in the past week as they battle to reverse rebel gains on a strategic highway, Human Rights Watch said on Sunday.
Rebels surrounded an army garrison on Sunday near a northwestern town, in the latest push to seize more territory in a province near the Turkish border, opposition activists said. Rebels also posted video on the Internet purportedly showing a fighter jet they had shot down in the area the previous day.
Several hundred soldiers were trapped in the siege of a base in Urum al-Sughra, on the main road between the contested city of Aleppo, Syria's commercial and industrial hub, and Turkey.
"Rebels attacked an armoured column sent from Aleppo to rescue the 46th Regiment at Urum al-Sughra and stopped it in its tracks," Firas Fuleifel, one of the activists told Reuters by phone from Idlib province, west of Aleppo. He said the jet was shot down while trying to provide air support to the column.
Rebels say they have been extending their control on the rugged agricultural province throughout the past week, capturing several towns on the border and making gains in the al-Rouge plain west of the city of Idlib, the provincial capital.
The province is the main base and supply route for rebels fighting urban warfare against Assad's forces for control of Aleppo, a city of several million people that could determine the course of the 18-month rebellion against Assad.
New York-based Human Rights Watch said cluster bombs were dropped from planes and helicopters near the main north-south highway running through Maarat al-Numan, a town rebels seized last week cutting the route from Damascus to Aleppo.
HRW previously reported Syrian use of cluster bombs in July and August, but the renewed strikes indicate the government's determination to regain strategic control in the northwest.
Cluster munitions drop hundreds of bomblets on a wide area, designed to kill as many people as possible. Human rights groups say their use near civilian homes can be a war crime.
More than 100 nations have banned their use under a convention which became international law in 2010, but Syria has not signed it, nor have Russia, China or the United States.
Towns targeted included Maarat, Tamanea, Taftanaz and al-Tah. Cluster bombs have also been used in other areas in Homs, Aleppo and Latakia provinces, and near Damascus, HRW said.
"Syria's disregard for its civilian population is all too evident in its air campaign, which now apparently includes dropping these deadly cluster bombs into populated areas," said Steve Goose, arms director at HRW.
HRW said it learned initially about the latest use of the weapons from videos released by opposition activists and had confirmed it in interviews with residents in two towns. It had no information on casualties. The bombs were Russian-made, but it was not known how or when Syria acquired them, it said.
Residents from Taftanaz and Tamanea - both near Maarat al-Numan - told HRW interviewers that helicopters had dropped cluster munitions on or near their towns last Tuesday.
One that hit Tamanea scattered bomblets between two schools, a resident was quoted as saying in the HRW report. People were taking away unexploded bomblets as souvenirs.
"The cluster munition strikes and unexploded ordnance they leave behind pose a huge danger to civilian populations, who often seem unaware how easily these submunitions could still explode," HRW's Goose said.
Syrian government officials were not immediately available to comment on the HRW report. The official state news agency said on Sunday that loyalist forces had killed dozens of "terrorists" in Aleppo, and had captured rockets.
The United Nations peace envoy for Syria, Lakhdar Brahimi, was due in Tehran later on Sunday for talks with Iranian officials, Iranian media reported. Brahimi, who took over the mediator job after former U.N. Secretary-General Kofi Annan quit in frustration, will meet President Mahmoud Ahmadinejad.
Shi'ite Iran is the main ally in the region of Assad, a member of the Alawite sect, an offshoot of Shi'ite Islam. U .N. Secretary-General Ban Ki-moon said this week Brahimi would visit Syria soon to urge Assad to call a ceasefire.
The anti-Assad uprising has been led by the Sunni Muslim majority and is backed by Sunni-ruled Arab states and by Turkey, also led by a party with its roots in Sunni Islamist politics. NATO member Ankara has increasingly taken on a leadership role in the international coalition ranked against Assad.
Turkish confrontation with Syria increased in the past two weeks because of cross-border shelling and escalated on Oct. 10 when Ankara forced down a Syrian airliner en route from Moscow, accusing it of carrying Russian munitions for Assad's military.
Russia has said there were no weapons on the plane and that it was carrying a legal shipment of radar equipment. The incident threatens to cause a crisis in strategically important Russian-Turkish relations after both countries worked to keep disagreement over Syria from ruining wider ties.
Syria said on Saturday it was banning flights by Turkish aircraft over its air space. The move was largely symbolic, as Turkey has already told its airlines not to fly over Syria because of the risk of retaliation.
In remarks indicating criticism of Moscow, which has blocked U.N. Security Council resolutions on Syria, Turkish Prime Minister Tayyip Erdogan told a conference in Istanbul on Saturday: "The U.N. Security Council has not intervened in the human tragedy that has been going on in Syria for 20 months.
"There's an attitude that encourages, gives the green light to, Assad to kill tens or hundreds of people every day."
The bloodshed has worsened markedly in the past two months although neither side has been able to gain a distinct advantage. Combat has been reported nationwide but the crucial strategic battles are being fought in an arc through western Syria, where most of the population lives.
Assad's forces still control the city of Idlib on a main highway linking Aleppo to the port of Latakia, making the route an important rebel target.
"Lots of roadblocks of Idlib have been taken out. Rebel focus is now on supplying the Aleppo highway," said Abu Ali, an activist using an alias.
After four days of heavy fighting in the town of Azmarin on the Turkish border, the rebels appeared to have a fragile hold.
"Praise be, the town is now in our hands ... We have raised two flags inside the town and the battles are over. Azmarin is completely under our control," one resident, who did not want to be named, told Reuters by telephone from inside the town.
A few kilometres (miles) along the border, clashes continued in the Syrian town of Darkush, where the crack of gunfire and sporadic sound of shelling could be heard from Turkey.