ISTANBUL — Turkish border guards killed at least 11 Syrian refugees, including at least three children, who were trying to cross into Turkey illegally on Sunday, Syrian activists and a monitoring group said.

The refugees were shot as they tried to enter Turkish territory near the border crossing of the Syrian town of Jisr al-Shoughour, the Syrian Observatory for Human Rights, monitoring group based in Britain, said in a statement on its website. Several other refugees were wounded, the group said.

Jisr al-Shoughour has become a key smuggling route since Turkey closed its borders to Syrian refugees this year.

A senior Turkish official said that the killings could not be independently verified and that an investigation was underway. Turkey, which is a host to nearly three million refugees, has denied similar previous allegations and maintains that it allows entry to refugees whose lives are under imminent threat.