Tanks stationed at a Turkish army position near the Oncupinar crossing gate close to the town of Kilis fire toward the Syria border on Feb. 16. (Bulent Kilic/AFP/Getty Images)

Turkey continued its bombardment of Kurdish forces inside northwestern Syria Tuesday, and expressed “disappointment” in a U.S. response that called on both sides to end hostilities.

U.S. support for the Syrian Kurds, Turkey’s ambassador to Washington Serdar Kilic said in an interview, “is a big strategic mistake . . . it will be regretted, but it will be too late.” The Obama administration, Kilic said in an interview, had put NATO member Turkey and the Kurdish Democratic Union Party, or PYD, “on an equal footing — an ally and a terrorist organization.”

Tensions grew between the United States and Turkey as new challenges arose to a ceasefire agreement reached late last week with Russia and other outside stakeholders in Syria’s civil war.

A meeting chaired by the United States and Russia to arrange for a cessation of hostilities by Friday failed to take place as scheduled Tuesday. At the same time, international condemnation rose in response to charges that Russian bombers struck hospitals and other civilian facilities Monday, killing at least 50 people in and around the city of Aleppo.

Russia denied that its jets were responsible; the Syrian government it backs suggested the strikes were conducted by U.S. warplanes.

A separate part of the agreement, calling on combatants to allow “immediate” humanitarian access to besieged civilian towns and cities in Syria, was also delayed. In an emergency visit to Damascus, U.N. envoy Staffan de Mistura obtained agreement from the government of President Bashar al-Assad to allow aid convoys to enter six besieged areas, a U.N. spokesperson said, and de Mistura suggested that assistance could begin to arrive on Wednesday.

Cross-border Turkish shelling of the PYD’s military forces has added a new dimension to the multi-party fighting in the strategic area between Aleppo and the Turkish border. Turkey considers the PYD to be one and the same as the Kurdistan Worker’s Party, a Turkish separatist group that it and the United States have designated as a terrorist organization.

But the administration has formed an alliance of convenience with the PYD, whose military forces have been useful in the fight against the Islamic State in Syria.

With the help of U.S. airstrikes and air-dropped weaponry, the Syrian Kurds, called the People’s Protection Units, or YPG by its Kurdish acronym, last year cleared the Syria-Turkey border east of the Euphrates River of Islamic State militants. It was the first significant ground victory in the fight against the Islamic State in Syria, and the administration has looked forward to continuing advances by the group.

In response to Turkish sensitivities, however, administration officials pledged to hold the group back from continuing a march westward, across the Euphrates, which would allow it to control the entire 500-mile border.

Turkey now says that pledge has been violated, with YPG forces attempting to take the town of Azaz, along a key opposition supply route four miles from the border, north of Aleppo. The Turks began shelling Kurdish positions Saturday, in response to what they said were artillery strikes on Turkish territory.

Over the weekend, State Department spokesman John Kirby urged both Turkey and the Syrian Kurds to focus on the “common threat” of the Islamic State instead of attacking each other. While urging the Syrian Kurds not to “take advantage of a confused situation,” Kirby imploredTurkey to “cease” firing across the border.

“There is a tremendous disappointment on the part of the Turkish authorities, as well as Turkish public opinion,” Kilic said. “Statements by the U.S. authorities have been misleading the PYD and other elements, who think they have a strong ally in the United States and can do what they want in terms of achieving their objectives.”

In Istanbul, an unnamed Turkish official who briefed reporters Tuesday said that Turkey “wants a ground operation” in Syria, without which, the official said, “it is impossible to stop this war.”

It was unclear whether the official was referring to the insertion of outside ground forces into the civil war, or the fight against the Islamic State. The two conflicts, long separate, have begun to converge in the area between Aleppo and the border.

Last week, Turkey and Saudi Arabia said they were willing to send ground forces to participate in anti-Islamic State operations, but only as part of a larger plan by the U.S.-led coalition against the militants. A unilateral action by the two Sunni Muslim countries would likely exacerbate the increasingly sectarian nature of the civil war.

A small contingent of about 50 U.S. Special Operations forces is already operating inside Syria — coordinating with the Syrian Kurds and Sunni opposition forces — and the administration has called on other nations to contribute.

But Kilic said Turkey has begun to despair that the United States is willing to show “more leadership, more muscle” in Syria, especially in the face of Russian bombing. The administration, he said, “didn’t respond decisively” when Russia annexed the Crimea from Ukraine, or when government forces used chemical weapons against Syrian civilians. “We are losing,” he said of the situation in Syria.