Ammon News - By Suleiman Al-Khalidi

AMMAN (Reuters) - Jordan has turned away thousands of Syrian refugees in the past week in the first such clampdown since the crisis in Syria began more than two years ago, diplomats, activists and aid workers said on Tuesday.

Jordan, due to host an international conference on Syria on Wednesday, has already taken in 473,587 Syrians out of a total of 1.5 million who have fled the conflict in an exodus that has accelerated in the past four months, U.N. figures show.

All four unofficial crossing points used by refugees trying to escape bombardments in the southern province of Deraa have been closed for the past six days, refugees and aid workers say, although the official frontier post at Jaber remained open.

They said Syrian families trying to pass into Jordan from the rebel-held border villages of Nasib and Tel Shehab had been turned away with no reason given by the Jordanians.

"The Jordanian authorities have stopped receiving refugees whatever their circumstance, except the wounded," Abu Hussein al-Zubi, a Syrian aid worker contacted by phone in Nasib, where he said at least 1,000 refugees were stranded.

"There are now many refugees gathering on the border trying to enter Jordan and waiting for the border to open," he said.

The daily exodus is facilitated by Syrian rebels and Jordanian troops on either side of a border marked only by a barbed wire barrier and Jordanian sentry towers.

A Western diplomat linked the closure to security measures before Wednesday's "Friends of Syria" meeting in Amman, where foreign ministers of Western and Gulf states opposed to Assad will discuss the quest for a political solution in Syria.

"The Jordanians are worried about security issues (and) are sending signals to the international community highlighting the huge refugee burden they are now shouldering," he said.

Resource-poor Jordan has long sought to win more outside help in its struggle to cope with the vast influx of refugees.

In Geneva, a U.N. official said it was important that Jordan keep its borders open to refugees.

"Of course we are discussing with everybody, we are discussing with the Jordanian government to make sure that people are able to cross without facing any difficulty to reach safety wherever they are," Panos Moumtzis, regional coordinator of the U.N. refugee agency UNHCR, told a news briefing.

UNHCR's representative in Jordan, Andrew Harper, confirmed to Reuters that the refugee flow had slowed to a trickle in the last few days, but said the reasons were not clear.

"What we are pretty sure of is that the refugees are not coming to the border. At the moment the access routes to the border are closed," he said.

Harper said fewer than 30 refugees had arrived in the last three days, compared to the usual 1,000 to 2,000 a day, but said Jordan had told UNHCR it was not turning back refugees.

"The Jordanians are saying the flow is restricted on the other side ... and that people are not actually getting to the border, whether that is true or not," he said.

Jordanian officials have made no public comment.

U.N. agencies say privately they cannot verify what happens at the border because they only register refugees when they arrive at Zaatari camp, which houses more than 100,000 people and is by far the largest Syrian refugee camp in the region.

Harper said much more needed to be done to expand relief efforts inside southern Syria to relieve the burden on Jordan, where resources are stretched to the utmost. (Additional reporting by Stephanie Ulmer-Nebehay in Geneva; Editing by Alistair Lyon)