Washington — As Turkish President Recep Tayyip Erdogan prepares to visit Washington for a White House meeting Wednesday, President Trump's national security adviser said he's "very concerned" about reports of potential war crimes in Syria in the wake of the Turkish-led invasion of territory formerly held by U.S.-allied Kurdish forces.

"We're very concerned about those issues, the war crimes issues," National Security Adviser Robert O'Brien said on "Face the Nation" Sunday. "We're watching them. We're monitoring it very closely. There is no place for genocide, for ethnic cleansing, for war crimes in the 21st century. The U.S. won't stand by for it, and we've made that position very clear to the Turks."

After Mr. Trump ordered an abrupt withdrawal of the U.S. troops stationed in northern Syria, the Turkish military launched a full-blown offensive with the help of Arab militias to take land held by the Kurds, which the government in Ankara has long accused of fostering discord and terrorism within Turkey.

Mr. Trump's announcement fueled a flurry of bipartisan outcry in Congress, with even typically staunch allies like Senator Lindsey Graham urging him to reverse course. Democrats and Republican alike warned Mr. Trump that an American draw-down from the region would effectively sanction a Turkish slaughter of Kurdish groups, which were instrumental in U.S. efforts to eradicate the territorial ISIS caliphate.

Reports quickly emerged of possible atrocities committed by Turkish-backed militias. Since the incursion, an uneasy ceasefire has stopped large-scale fighting, with the Turks controlling some of the land they seized, and Russian and Syrian forces jointly patrolling other previously Kurdish-held territory.

O'Brien said Sunday that he has seen "very disturbing" reports since the Turkish-led invasion, but maintained that the Turkish government has assured the Trump administration that it is investigating them.

Asked whether the White House would sign-off on sanctions against Turkey if Congress would authorize them, O'Brien demurred, pointing instead to the president's meeting with Erdogan on Wednesday. Despite acknowledging that the administration is "upset" with Turkey over its recent foreign policy moves, including the purchase of Russian-made anti-aircraft missiles, O'Brien underscored the geopolitical importance of U.S.-Turkish relations.

"Losing Turkey as an ally is not something that's good for Europe or for the United States," he added.