Turkey, Jordan and Iraq must fully reopen their border crossings to allow thousands of Syrians fleeing their country's war to seek refuge, Human Rights Watch said on Monday.

"Iraqi, Jordanian, and Turkish border guards are pushing back tens of thousands of people trying to flee Syria," the international rights group said, adding that the lives of those trying to flee were in danger.

"Iraq, Jordan, and Turkey have either closed numerous border crossings entirely or allowed only limited numbers of Syrians to cross, leaving tens of thousands stranded."

The U.N. says more than 1.7 million Syrians have fled the conflict in their country. The vast majority have sought refuge in neighboring countries.

But only Lebanon has kept an open-door policy for Syrian refugees, says HRW.

Syria's war broke out more than 27 months ago and has left more than 100,000 dead, the Syrian Observatory for Human Rights watchdog says.

New York-based HRW stressed international law prohibits countries "from sending anyone back to -- or pushing back anyone trying to leave -- a country where their life or freedom would be threatened".

HRW urged donor countries to step up support to countries hosting Syrian refugees.

But "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," HRW stressed.

It said Jordan denies closing its borders, though "recently arrived refugees... say that Jordanian border guards blocked their and others' entry for days or weeks in May".

Iraq's Kurdistan regional government has admitted closing the Syria border but has since mid-June allowed some Syrians in need of emergency assistance to cross over.

Baghdad has "severely limited the number of Syrians allowed to enter since August 2012, and new arrivals virtually ceased in late March," HRW said.

Turkey is "blocking the entry of thousands of Syrians at the Bab al-Salam, Atma, and other border crossings with Syria", it added.