* Refugee numbers have nearly doubled this year-UNHCR
* Many live in tents or crowded shelters
* U.N. says refugee aid plan only 25 percent funded
* No end in sight for Syria's civil war (Recasts with millionth refugee, adds details, quotes)
By Oliver Holmes
TRIPOLI, Lebanon, March 6 (Reuters) - A 19-year-old mother of two registered on Wednesday as the millionth refugee to flee Syria, part of an accelerating exodus that is piling pressure on neighbouring host countries.
Wearing a green headscarf and holding her young daughter, Bushra smiled nervously as she waited at Lebanon's main registration centre in the northern city of Tripoli, which processes 800 Syrians a day.
"The situation is very bad for us. We can't find work," she said. "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 nearly two years ago when President Bashar al-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.
"We are doing everything we can to help, but the international humanitarian response capacity is dangerously stretched. This tragedy has to be stopped."
In addition to the refugees, more than two million of Syria's 22 million people have been internally displaced and more than four million need humanitarian assistance, UNHCR says.
UNHCR said the number of Syrians quitting their country has increased dramatically since the beginning of the year with more than 400,000 - nearly half the total - leaving since Jan. 1.
Most have fled to Lebanon, Jordan, Turkey, Iraq and Egypt and some to North Africa and Europe, arriving traumatised, without possessions and having lost relatives, it said.
Syria's smallest neighbour Lebanon, whose population is only four million, hosts the highest number of refugees, with arrivals doubling to 4,400 a day in the past three weeks, UNHCR representative in Lebanon Ninette Kelley told Reuters.
Including Syrian workers and self-supporting Syrian families, one in five people in Lebanon is now Syrian.
Ghassam Mahmoud, a mechanic, had brought his two wives and 12 children from Lebanon's northern Akkar province to the Tripoli registration centre. He said he was sleeping in a tent after escaping Khan Sheikhoun, a Syrian town in Idlib province that has been shelled by Assad's military.
"I've looked for work (in Lebanon) but there is none. When it comes along, I'll do it," he said, adding that he had waited for two months to be registered due to the backlog.
Several hundred families, mostly women and children, sat behind Mahmoud waiting to be registered.
Sixteen offices made of wood and plastic have been built where the families are interviewed. Some have high walls to protect the identities of victims of sexual abuse.
Despite pledges of $1.5 billion by international donors for a U.N. response plan to help Syria's displaced, only 25 per cent has been funded, UNHCR said.
In Jordan, a country of six million, the refugee influx has strained energy, water, health and education services to the limit.
Turkey says it has spent more than $600 million setting up 17 refugee camps, with more under construction. But Fuat Oktay, president of the country's disaster and relief management body, AFAD, said on Wednesday the overall cost of caring for the refugees was closer to $1.5 billion.
There is no end in sight to the conflict in Syria, which has divided world powers. Russia and Shi'ite Iran support Assad, while the United States, along with some European and Sunni Muslim Gulf Arab nations back a fractured opposition.
Damascus and some of its opponents have said they will consider peace talks, but no meetings have been arranged. (Additional reporting by Dominic Evans in Beirut and Jonathon Burch in Anakara; Editing by Alistair Lyon)