(Adds U.N. reaction, report of air raid on northeastern Syrian city, North Caucusus Islamists join rebels)
* U.N. says 20 peacekeepers held, rebels want Assad pullback
* Same rebel brigade accused of past executions
* One million refugees have fled Syrian conflict
* Arab League offers opposition seat at next meeting
By Oliver Holmes
BEIRUT, March 6 (Reuters) - Syrian rebels have seized a convoy of U.N. peacekeepers near the Golan Heights and say they will hold them captive until President Bashar al-Assad's forces pull back from a rebel-held village which has seen heavy recent fighting.
The capture was announced in rebel videos posted on the Internet and confirmed on Wednesday by the United Nations, which said about 20 peacekeepers had been detained.
The seizure is the most direct threat to U.N. personnel in the nearly two-year-old uprising against Assad, and Human Rights Watch said it was investigating the same brigade for past executions.
It came on the day Britain said it would increase aid to the opposition forces and the Arab League gave a green light to member states to arm the rebels.
The Arab League also invited the opposition Syrian coalition to take Syria's seat at a meeting of the regional body in Doha later this month. Syria was suspended in November 2011 in response to its crackdown on protests which has since spiralled into civil war.
In the latest attack by the Syrian military, warplanes bombarded the northeastern provincial capital of Raqqa for a second consecutive day on Wednesday, killing at least 39 people, opposition activists said. Video footage showed fighters putting dismembered bodies in an ambulance.
The peacekeepers of the UNDOF mission have been monitoring a ceasefire line between Syria and the Israeli-occupied Golan Heights, captured by the Jewish state in a 1967 war, for nearly four decades.
Israel has warned that it will not "stand idle" as Syria's civil war spills over into the Golan region.
The United Nations in New York said its peacekeepers had been detained by around 30 fighters in the Golan Heights. The Security Council and Secretary-General Ban Ki-moon condemned the seizure of U.N. observers and demanded their immediate release.
"The U.N. observers were on a regular supply mission and were stopped near Observation Post 58, which had sustained damage and was evacuated this past weekend following heavy combat in close proximity at Al Jamla," the United Nations said, referring to a village which saw fierce clashes on Sunday.
It did not mention the nationality of the observers, but the Syrian Observatory for Human Rights monitoring group, which is in contact with the rebel brigade, said they were Filipino.
In one rebel video, a young man saying he was from the "Martyrs of Yarmouk" brigade stood surrounded by several rebel fighters with assault rifles in front of two white armoured vehicles and a truck with "UN" markings.
"The command of the Martyrs of Yarmouk ... is holding forces of the United Nations Disengagement Observer Force until the withdrawal of forces of the regime of Bashar al-Assad from the outskirts of the village of Jamla," said the man, who was wearing civilian clothes.
At least five people could be seen sitting in the vehicles wearing light blue U.N. helmets and bulletproof vests.
"If no withdrawal is made within 24 hours we will treat them as prisoners," the man said, accusing them of collaborating with Assad's forces to push the rebels out of Jamla.
Nearly two years after the uprising started, rebels are distrustful of the United Nations, which they say has failed to support their cause.
Earlier on Wednesday, the United Nations said the number of refugees who have fled Syria had reached 1 million, part of an accelerating exodus from a conflict which is approaching its second anniversary with no prospect of an end to the bloodshed.
British Foreign Secretary William Hague, pledging support for Assad's opponents, said the civil war had reached catastrophic proportions and that international efforts to stem the violence had been an abject failure.
Senior U.S. and Russian diplomats will discuss the conflict at a meeting in London on Thursday, Russia said, the latest in a series of meetings aimed at seeking an end to the fighting.
But Hague said the chances of getting an immediate political solution to the crisis were slim and that diplomacy was taking too long.
"If a political solution to the crisis in Syria is not found and the conflict continues, we and the rest of the European Union will have to be ready to move further, and we should not rule out any option for saving lives," he said.
However, Hague played down the prospect of direct Western military intervention.
While Moscow has been one of Assad's main protectors, members of an Islamist insurgency involved in daily clashes in Russia's predominantly Muslim North Caucasus and their compatriots have trickled into Syria to fight on the rebels' side.
A Syrian rebel leader sought to persuade European governments to lift an arms embargo for the rebels, saying any weapons provided would be accounted for and possibly returned.
"The weapons are registered on lists with numbers on each weapon. We distribute those weapons. And we know precisely who has received them," Brigadier Selim Idris told a news conference in Brussels.
ONE MILLION REFUGEES
At a registration centre for Syrians in the northern Lebanese city of Tripoli, a 19-year-old mother of two registered on Wednesday as the millionth refugee to flee her country.
"The situation is very bad for us. We can't find work," said the teenage mother, wearing a green headscarf and holding her daughter as she spoke to reporters.
"I live with 20 people in one room. We can't find any other house as it is too expensive. We want to return to Syria. We wish for the crisis to be resolved."
Syrians started trickling out of the country 23 months ago when Assad's forces shot at pro-democracy protests inspired by Arab revolts elsewhere.
The uprising has since turned into an increasingly sectarian struggle between armed rebels and government soldiers and militias. An estimated 70,000 people have been killed.
Around half the refugees are children, most of them aged under 11, and the numbers leaving are mounting every week, the United Nations refugee agency said in statement.
"With a million people in flight, millions more displaced internally, and thousands of people continuing to cross the border every day, Syria is spiralling towards full-scale disaster," U.N. High Commissioner for Refugees António Guterres said in a statement. (Additional reporting by Dominic Evans and Laila Bassam in Beirut and Jonathon Burch in Anakara; Editing by Michael Roddy and Mohammad Zargham)