(Adds Turkey's reaction)

BEIRUT, July 1 (Reuters) - Syria's neighbours have closed or tightened restrictions at several border crossings, leaving tens of thousands of people stranded within Syria's dangerous frontier regions, Human Rights Watch said on Monday.

It said Iraq, Jordan and Turkey had all restricted the flow of people trying to flee a conflict which has killed 100,000 people and, according to the United Nations, has already driven 1.7 million more to take sanctuary outside Syria.

Only Lebanon, which has limited control over its own borders and is now hosting over half a million refugees, had remained open to refugees, the New York-based rights group said.

"Iraq, Jordan, and Turkey risk turning Syria into an open-air prison for tens of thousands of Syrians unable to escape the carnage in their country," said Gerry Simpson, senior refugee researcher at Human Rights Watch.

"Neither the pressure those countries are under due to rising refugee numbers, nor giving aid inside Syria, can justify violating people's basic right to seek asylum from persecution and other abuse."

Officials in Iraq and Jordan could not immediately be reached for comment but a senior Turkish official rejected the rights group's assertion.

"There has been no changes in Turkey's open-door policy towards Syrian refugees ... No restrictions have been imposed regarding this matter," the official said on condition of anonymity, adding new camps to host more refugees were under construction.

"Because of security concerns, we might have closed one or two border gates but Syrians can use other entry points for crossing into Turkey."

HRW said Turkey was blocking thousands of Syrians at the Bab al-Salam, Atma and other crossings, and only sporadically allowing small numbers from camps inside Syria to cross.

HRW said the Iraqi authorities in Baghdad had severely limited the numbers of Syrians allowed to enter since August 2012, and that new arrivals virtually ceased in late March.

Iraq's semi-autonomous Kurdistan Regional Government closed its border with Syria in May, and since mid-June only some Syrians in need of emergency humanitarian assistance have been allowed to cross, it said.

In May, aid workers and refugees in Jordan told Reuters that thousands of Syrians were stranded on the border when Jordanian authorities turned them back.

"Syria's neighbours should stop pushing desperate people back to places where their lives are in danger," Simpson said, adding that international donors should help those countries shoulder the burden of hosting the continuing wave of refugees. (Additional reporting by Parisa Hafezi in Ankara; Editing by Alison Williams)

Our Standards: The Thomson Reuters Trust Principles.