Russian-controlled forces in Syria have been roaming deeper into U.S.-held territory, raising the risk of conflict in an apparent effort “to challenge our presence in the northeast,” according to a senior State Department official.

“These are not daily occurrences, but they have been increasing in number,” James Jeffrey, the State Department’s lead negotiator for the Syria crisis, told reporters on Wednesday. “It’s troubling.”

Those close encounters with American troops are just one accelerant in an already combustible scenario involving Russian-controlled, Iranian, and Syrian units and the Syrian Democratic Forces — the local Kurdish and Arab militias that partnered with the United States to defeat the Islamic State.

“They try to come deep into the area where the SDF are patrolling, well inside the basic lines that we have sketched,” Jeffrey said. “I will stress the professionalism of the troops on the ground, beginning with our troops, is such that these incidents have not escalated. But of course, any commander would be concerned about this, and we call upon the Russians to adhere fully to the deconfliction agreement.”

International efforts to end the civil war that has raged since 2011 remain stalled after negotiations over a committee to rewrite the Syrian Constitution were “frozen” in October. Jeffrey credited Russian diplomas with attempting to bring Assad into compliance with the demands leveled by the United Nations, which include pressing the regime to end human rights abuses and “dramatically change its relationship with Iran” and with Iran’s chief terrorist proxy, Hezbollah.

“Russia apparently feels that it cannot bring the regime to do the necessary things to bring it in line with the international community's expectations and needs, so the Russians are going to press forward for a military victory,” Jeffrey surmised. “Whether that reflects differences within the Russian government, because there are clearly elements of the Russian government, such as the military — they're very much all in on this awful offensive that is targeting, deliberately, civilians — or it could be problems in the triangle between Tehran, Moscow, and Damascus.”

Syrian forces shelled a Turkish military outpost, killing seven soldiers, on Monday. “Brutal actions by the Assad regime, Russia, the Iranian regime, and Hezbollah are directly preventing the establishment of a ceasefire in northern Syria,” Secretary of State Mike Pompeo said on Tuesday evening.

“We think that this is an extremely dangerous conflict,” Jeffrey said. “It needs to be brought to an end. Russia needs to change its policies."