The United States and Russia have called for tensions to de-escalate in northern Syria amid retaliatory strikes by Türkiye and the PKK terrorist group's Syrian branch YPG along the Syrian border.

The U.S. opposes any military action that destabilizes the situation in Syria, a State Department spokesperson said. Separately, Russia called on Türkiye to show restraint in its use of military force in Syria and to keep tensions from escalating.

The comments came after Türkiye said the U.S.-backed PKK/YPG killed three people in mortar attacks launched from northern Syria on Monday, following Turkish air operations over the weekend and a deadly bomb attack in Istanbul a week earlier.

A child and a teacher were among those killed and six people were wounded when mortar bombs hit a border area in Türkiye's Gaziantep province. Turkish forces responded with jets again hitting targets in Syria, a senior security official said.

President Recep Tayyip Erdoğan has said operations would not be limited to an air campaign and may involve ground forces. Türkiye has conducted several major military operations against the PKK/YPG and Daesh terrorists in northern Syria in recent years.

The U.S. State Department spokesperson said Washington had communicated its serious concerns to Ankara about the impact of escalation on the goal of fighting Daesh.

"We have urged Türkiye against such operations, just as we have urged our Syrian partners against attacks or escalation," the spokesperson said in emailed responses to questions.

"We continue to oppose any military action that destabilizes the situation in Syria or violates Iraq’s sovereignty through military actions uncoordinated with the Iraqi government. We also oppose the recent attacks into southern Türkiye that have reportedly led to the deaths of multiple civilians," the spokesperson said.

The United States has allied with the PKK/YPG in the fight against Daesh in Syria, causing a deep rift with NATO ally Türkiye.

Russia on Tuesday called for Türkiye to exercise "restraint" and warned against "destabilizing" Syria. "We understand and respect Türkiye's concerns regarding its own security. We believe this is the legal right of Türkiye," Kremlin spokesperson Dmitry Peskov told reporters.

"We still call on all parties to refrain from steps that could lead to seriously destabilizing the situation," he said. He added that it could "boomerang back and further complicate the security situation".

"We will call on our Turkish colleagues to show certain restraint in order to prevent an escalation of tension, not only in the northern and northeastern regions of Syria but throughout the entire territory," the envoy, Alexander Lavrentyev, also said.

Lavrentyev said Türkiye had not notified Russia in advance about its strikes on Syria and Iraq.

"We hope to convince our Turkish partners to refrain from using excessive force on Syrian territory," Lavrentyev said. He added that Russia would work with interested parties to find a peaceful solution to the issue.

Lavrentyev said that the withdrawal of a U.S. contingent from the northeast of Syria would contribute to the "stabilization" of the situation.

"Russia has for months ... done everything possible to prevent any large-scale ground operation," Lavrentyev said in the Kazakh capital, which is hosting a tripartite meeting between Russia, Türkiye and Iran on Syria.

The three countries are major players in the war in Syria, which has claimed nearly half a million lives since 2011.

Asked if Türkiye discussed this operation with Russia and the United States, Erdoğan said Monday: "We have not had any conversations with (U.S. President Joe) Biden or (Russian President Vladimir) Putin regarding this operation. However, both Biden and Putin already know that we can do such things in this region at any time."

Erdoğan recalled the Sochi deal between Türkiye and Russia to remove PKK/YPG terrorists from northeastern Syria bordering Türkiye.

"They had the responsibility to clear the terrorists from the area. Unfortunately, even though we reminded them many times over and over, they did not do this.

"We said that we would not remain silent against this and that we would take steps against the terrorists there if they could not do it," he said.

Türkiye is with the United States in NATO, Erdoğan also said, noting, however, that Washington has unfortunately sent thousands of equipment, ammunition and weapons to the terror zones in Syria.

During his talks with Biden in Indonesia, Erdoğan said: "I told him: 'We are together with you in NATO, we are two important allies. Since such a threat came to us from the south, you are putting us in danger by giving support to the terrorist organizations here. Of course, we cannot live with these problems. When necessary, we will have to give the necessary answer to them.' Of course, they can't say anything. We have done what is necessary at this point, we are doing it, and we will continue to do so."

The PKK is a designated terrorist organization in the U.S., Türkiye and the European Union and Washington's support for its Syrian affiliate has been a major strain on bilateral relations with Ankara. The PKK/YPG has controlled much of northeastern Syria after the forces of Syrian regime leader Bashar Assad withdrew in 2012. The U.S. primarily partnered with PKK/YPG terrorists in northeastern Syria in its fight against the Daesh terrorist group. On the other hand, Türkiye strongly opposed the PKK/YPG's presence in northern Syria.

Under the pretext of fighting Daesh, the U.S. has provided military training and given truckloads of military support to the PKK/YPG, despite its NATO ally's security concerns. Underlining that one cannot support one terrorist group to defeat another, Türkiye conducted its counterterrorism operations, throughout which it has managed to remove a significant number of terrorists from the region.

Erdoğan has said as the U.S. and Russia failed to live up to their commitments to provide such a safe zone in the border region. In October 2019, Russia expressed commitment to removing the terrorist group from Tal Rifaat and Manbij after reaching an agreement with Türkiye during Operation Peace Spring. Moscow also promised that the terrorists would be removed 30 kilometers (19 miles) from the border on the M4 road and in the area outside the Operation Peace Spring area. Likewise, then-U.S. Vice President Mike Pence pledged to Türkiye that the YPG/PKK terrorist group would withdraw from the region of Operation Peace Spring.

Reaction from Germany

Meanwhile, Germany on Monday urged a "proportional" response in compliance with international law after Türkiye carried out airstrikes against bases of PKK/YPG militants across northern Syria and Iraq.

"Civilians at all times must be protected," German Foreign Ministry spokesperson Christofer Burger told a government briefing, describing reports of civilian victims in the Turkish air raids as "extremely worrying."

"We call on Türkiye to react proportionally and to respect international law," he added.

Burger said Germany urged, "Türkiye and all other participants to do nothing that would aggravate the already tense situation in northern Syria and Iraq."