* Bombing of camp first such attack on a refugee camp near Jordanian border since 2011 conflict
* UNHCR says the attacks shows no areas safe in Syria, appeals to Jordan to keep borders open
* Jordan tightens entry of Syrian refugees as country says cannot cope with huge influx (Adds more details, UNHCR comments)
By Suleiman Al-Khalidi
AMMAN, June 18 (Reuters) - Syrian army helicopters killed at least 20 people, mostly children and women, in the first attack on a refugee camp in southern Syria along the border with Jordan, residents and opposition activists said.
They said the army dropped several barrel bombs - highly destructive improvised explosives, use of which has been condemned by Western powers as a war crime - on the camp in the village of Shajra, 2 km (1 mile) from the Jordanian border.
"Women were wailing hysterically as they saw their dead children lying on the floor," said Abu Mohammad al-Hourani, a farmer in the village who said he had helped to remove bodies after the dawn raid. At least eighty people were injured, many seriously, aid workers said.
Hundreds of families fleeing intensified fighting between rebels and the army in southern Syria in recent months had taken shelter in the town near the border, which Jordan closed last year after taking in hundreds of thousands of Syrian refugees.
Witnesses said the attack led to hundreds of panic stricken families fleeing the rural border town of over 15,000 people for fear of further aerial attacks on the camp that was set up nearly five months ago.
The difficulties of crossing into Jordan, which says it is overburdened with more than 600,000 U.N. registered refugees, almost 15 percent of its population, has led some NGO and relief agencies to consider constructing camps inside Syrian territory close to the border.
The bombing of the refugee camp, where hundreds of families are living in miserable conditions, has not only shocked NGOs and aid workers operating in Jordan but lead to a reassement of the risks of having such camps in what was considered relatively safe areas close to the border, aid workers say.
But the UN's refugee aid agency UNHCR, which has long discouraged such camps inside Syria without international blessing, said host countries such as Jordan and Lebanon should continue to allow refugees fleeing the escalating violence in Syria to find a safe haven on their territory.
"They are very few areas in Syria that are safe, that is why it is important that countries continue to keep their borders open because establishing camps in dangerous locations does not provide protection to people," Andrew Harper, head of the UNHCR's Jordanian operations told Reuters.
"If you set up a camp for people who need protection, you need a protected place. People fleeing violence must at least be able to cross the border safely," he added.
Jordanian authorities now allow only a trickle of refugees into Jordan from a U.N.-funded border crossing in a desolate area close to the frontier with Iraq, where relief agencies say they have to endure days of hardship before arriving.
Amman also imposed this month restrictions on entry of ordinary Syrians who arrive through its airport and official border crossing with Syria, official sources say.
More than 160,000 people have been killed in Syria's conflict, which started in March 2011 with peaceful protests against President Bashar al-Assad's rule and became an armed rebellion after a government crackdown.
(Reporting by Suleiman Al-Khalidi; Editing by Ralph Boulton)