The White House refused to comment on Monday on reports of a deadly US military strike in Syria near the border with Iraq that has outraged Damascus and other Arab countries.
"I'm not going to comment on it at all," White House spokeswoman Dana Perino said.
The Syrian government and Arab leaders in the region have expressed anger over the purported US helicopter raid Sunday targeting militants in the town of Abu Kamal.
The Syrian government says civilians, including children, were among the eight dead.
The US has accused Syria of failing to halt the flow of Al Qaeda-linked and other militants into Iraq since the 2003 invasion.
US State Department spokesman Sean McCormack confirmed the Syrian Foreign Ministry summoned the US charge d'affaires at the embassy in Damascus, Maura Connelly, "to raise with her these reports of activities in Abu Kamal".
The US withdrew its ambassador from Syria in 2005.
McCormack said the number of militants entering Iraq through Syria has declined over the years.
"That isn't to say that there aren't continuing issues there, in terms of people coming over the border," he said.
The Syrian government strongly condemned the raid.
"We consider this an act of criminal and terrorist aggression," Syrian Foreign Minister Walid al-Muallem said.