* Crisis will not be confined to Syria indefinitely - Brahimi says
* Appeals for temporary Eid truce as "small step" to end bloodshed
* France says Assad's air force bombing from high altitude
The international mediator on Syria said on Wednesday its civil war risks spilling across borders to engulf the Middle East and appealed for a temporary truce he said could mark a small step towards defusing 19 months of conflict.
Lakhdar Brahimi, the U.N.-Arab League envoy, has proposed that both President Bashar al-Assad's forces and rebel fighters seeking his overthrow hold fire during the Islamic feast holiday of Eid al-Adha that starts next week.
Syrian authorities, who blame rebels for the failure of an April ceasefire plan, guardedly welcomed Brahimi's proposal but said any initiative must be respected by both sides.
Thirty-thousand people have been killed in the uprising, which began with peaceful demonstrations and now pits mainly Sunni Muslim rebels against an Alawite president. There are fears of broader Middle East sectarian conflict between Sunni powers sympathetic to the rebels and Shi'ites who back Assad.
"This crisis cannot remain within Syrian borders indefinitely. Either it will be addressed or it will increase ... and be all-consuming," Brahimi told reporters in Beirut after talks with Lebanese leaders.
The British-based Syrian Observatory for Human Rights said at least 90 people had been killed in Syria by late afternoon on Wednesday, after 150 people died the day before.
The death toll has topped 1,000 a week for at least two months as divided world powers have condemned the bloodshed in what has become a largely stalemated conflict, but failed to agree on a political solution.
On Sunday, Brahimi appealed to leaders in Iran - Assad's strongest regional ally - to support a proposal for a ceasefire to mark Eid al-Adha, expected to begin at dusk on Oct. 25.
Syrian officials have questioned whether the disparate rebels, who agreed on a joint leadership on Tuesday to encourage supporters to provide them more powerful weapons, could commit to or honour any ceasefire deal.
But Brahimi said opposition figures had told him any ceasefire by Assad's forces would be reciprocated immediately.
"We heard from everyone we met in the opposition, and everyone (else) we met that, if the government stops using violence, 'We will respond to this directly'," he said.
"We hope this will be a very small step that would save the Syrian people ... because they are burying hundreds of people every day."
Anti-Assad activists posted videos on Wednesday of what they said was a Syrian military helicopter spiralling to the ground and exploding in flames.
Rami Abdelrahman, head of the Syrian Observatory for Human Rights, said that the helicopter had been downed near Maarat al-Numan. "Some rebels say they used anti-aircraft missiles," he told Reuters by telephone from Britain.
Amateur footage of rebels using shoulder-mounted surface-to-air missiles have emerged in the past few days, and France's Foreign Minister Laurent Fabius said that heavy weapons acquired by the insurgents have forced Assad's air force to bomb rebel-held areas from high altitude.
"In a certain number of these zones, Bashar al-Assad is bombarding them with MiG fighter jets, and what is particularly horrible is that he is bombarding them with TNT," Fabius said.
"But at the same time there are now weapons that are forcing the planes to fly extremely high, and so the strikes are less accurate," he said before meeting civilian members of rebel councils that run areas outside government control.
France began channelling money and humanitarian aid to rebel-held parts of Syria in August to support self-rule and try to create an alternative to the Damascus government.
However, the French plan falls well short of the foreign-protected safe havens the opposition says it needs and offers little hope of relief to civilians fleeing the chaos.
Russia, which sold Syria arms worth $1 billion last year, and China have vetoed three resolutions favoured by Western powers condemning Syrian authorities and opening the way to U.N. sanctions on Damascus.
Fabius said Moscow's stance would only cement chaos in Syria, adding that he had told Russian Foreign Minister Sergei Lavrov that: "If you continue to oppose a change of regime, then the extremists risk taking control."
Russia denies trying to prop up Assad, who allows Russia to maintain a naval supply facility in the port of Tartus that is its only military base outside the former Soviet Union.
But Moscow says Syria's crisis must be resolved without foreign interference, particularly military intervention.
Syrian Foreign Ministry spokesman Jihad Makdissi said Damascus hoped Brahimi's talks in the region, including with countries which back the rebels, could herald "something which leads to the success of a constructive initiative".
Makdissi said Syria had previously committed to Arab and U.N. initiatives but that they had been thwarted by "armed groups and the countries that influence them".