* Syrian forces accused of using cluster munitions
* Rebels claim to have shot down jet
* U.N. envoy Brahimi in Iran for talks - Iran media
By Angus MacSwan and Khaled Yacoub Oweis
BEIRUT/AMMAN, Oct 14 (Reuters) - Turkey has banned all Syrian aircraft from its air space as it takes an increasingly firm stance against President Bashar al-Assad, while Syrian rebels said on Sunday they had made more gains in a key province near the Turkish border.
Human Rights Watch said Syrian government forces had dropped Russian-made cluster bombs over civilian areas in the past week as they battled to reverse rebel advances, an act which rights groups say can constitute a war crime.
NATO-member Turkey 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.
Turkish Foreign Minister Ahmet Davutoglu said on Sunday Turkish air space had been closed to Syrian planes. Syria banned Turkish planes from flying over its territory on Saturday.
"We made a new decision yesterday and informed Syria. We closed our air space to civilian Syrian flights as well as military flights," Davutoglu said.
The bloodshed inside Syria 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.
Rebels surrounded an army garrison on Sunday near a northwestern town, in the latest push to seize more territory near the border with Turkey, 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 of 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.
After four days of heavy fighting in the town of Azmarin and surrounding villages along the border with Turkey's Hatay province, the rebels appeared to have a fragile hold there.
"These areas are the last areas around the border where Assad has control. If he loses these then all of the border around Hatay will be under the control of the (rebel) Free Syrian Army," rebel fighter Ahmad Qasem told Reuters, after crossing into Turkey.
"Assad's army is taking its last breath in this area."
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.
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. More than 100 nations have banned their use under a convention which became international law in 2010, but Syria has not signed it, nor has 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.
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 in Tehran on Sunday. Brahimi, who took over the mediator job after former U.N. Secretary-General Kofi Annan quit in frustration, will meet Foreign Minister Ali Akbar Salehi.
"We think that these problems can be resolved peacefully. The United Nations and the Arab League are prepared to offer any assistance in resolving this crisis," Brahimi told a news conference after the meeting. "The U.N. is ready and demands a stop to the sending of arms to all groups in Syria."
Salehi said Iran was ready to work with Brahimi for peace and repeated its call for an immediate ceasefire before reforms and elections to resolve the conflict.
"We all need to join hands so that this conflict comes to a halt and further bloodshed is stopped," Salehi said.
Shi'ite Iran is the main ally in the region of Assad, who is 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.
Brahimi met with Turkish President Abdullah Gul, Davutoglu and Syrian opposition members in Istanbul on Saturday.