ISTANBUL (Reuters) - Turkish guards at the border with Syria are indiscriminately shooting at and summarily returning asylum seekers attempting to cross into Turkey, Human Rights Watch said on Saturday.

The spokesman for Turkey's president said it was highly unlikely that Turkish soldiers would shoot at Syrian war refugees but the government would look into the report. Turkey has taken in 3.5 million Syrian war refugees since 2011.

New York-based Human Rights Watch said Syrians were now fleeing heightened violence in the northwestern province of Idlib to seek refuge near Turkey's border, which remains closed to all but critical medical cases.

Syrian armed forces have thrust deeper into the mainly rebel-held province in recent months and Turkey last month launched military action in the nearby Afrin region targeting Kurdish YPG militia fighters.

"Syrians fleeing to the Turkish border seeking safety and asylum are being forced back with bullets and abuse," said Lama Fakih, deputy Middle East director at Human Rights Watch.

Asked about the HRW statement, President Tayyip Erdogan's spokesman said Turkish soldiers were there to protect these people and that Ankara has had an "open-door policy" since the start of Syria's civil war in 2011.

"We will certainly look into it. It's very unlikely that our soldiers would shoot at people," Ibrahim Kalin told reporters in Istanbul. "We never ask anyone whether they are Kurdish or Arab or Muslim Christian, from this region or that region. I don't know if it's an isolated incident or not."

HRW cited U.N. figures saying 247,000 Syrians were displaced to the border area between Dec. 15 and Jan. 15, 2018.

"As fighting in Idlib and Afrin displaces thousands more, the number of Syrians trapped along the border willing to risk their lives to reach Turkey is only likely to increase," Fakih said.

Thirteen of 16 Syrian refugees to whom HRW spoke accused Turkish border guards of having shot toward them or other fleeing asylum seekers as they tried to cross while still in Syria, killing 10 people, including one child, and injuring several more.

Turkey has taken in more Syrian refugees than any other country, granting many temporary protection status and providing them with basic services, including medical care and education.

"However, Turkey's generous hosting of large numbers of Syrians does not absolve it of its responsibility to help those seeking protection at its borders," the HRW statement added.

It said Erdogan's government should issue standard instructions to border guards at all crossing points that lethal force must not be used against asylum seekers and that no asylum seeker is to be mistreated.

(Additional reporting by Dominic Evans; Writing by Daren Butler; editing by Mark Heinrich)