Turkey's president Erdogan says campaign against Kurds in Syria will continue

Turkey's president has said his country will "not take a step back" from its military operation on an enclave in northern Syria controlled by U.S.-allied Kurdish fighters.

Speaking in Ankara on Monday on the third day of the operation, Recep Tayyip Erdogan said Turkey's "fundamental goal" is ensuring national security, preserving Syria's territorial integrity and protecting the Syrian people.

"We discussed this with our Russian friends, and we have an agreement," he said. Moscow, a military ally of Syrian leader Bashar al-Assad that operates a major air base in Syria, has not confirmed giving a green light to Turkey's campaign, to which Syria has strongly objected.

Erdogan slammed the United States for working with Syrian Kurdish YPG forces instead of Turkey in combatting the Islamic State or Iraq and Syria (ISIS). The U.S.-backed Syrian Democratic Forces (SDF) are dominated by YPG fighters.

The operation's aim, according to Erdogan, is not to "occupy" any part of Syria but rather to conquer "hearts." Ankara considers the YPG a terrorist group tied to Kurdish insurgents in Turkey.

Once Afrin and Idlib to the west are secured, Erdogan said hundreds of thousands of Syrians could return to their homes.

Erdogan said: "The Afrin operation will end when it reaches its goals."

The death toll from the offensive in the Afrin region stands at 18 civilians, including women and children, the spokesman for the Syrian Democratic Forces (SDF) militia alliance said on Monday. An additional 23 people have been wounded in the offensive, SDF spokesman Kino Gabriel added in a statement circulated on an instant messaging group run by the SDF.

Defense Secretary Jim Mattis, speaking from Indonesia, said Turkey gave the U.S. military advance notice of its airstrikes against Kurdish targets in northern Syria.

Mattis defended Turkey, calling it a trusted NATO ally with "legitimate security concerns" about Syria.

Deteriorating relationship

Mattis said diplomats are working on a solution to Turkey's armed confrontation with Syrian Kurds. He said the U.S. forces based in Syria had not been put at risk by the Turkish attacks, since U.S. troops are not based in that part of Syria.

In a statement released Monday afternoon, the White House said the operation "risks exacerbating the humanitarian crisis" in Syria.

"We ask that Turkey ensure its operations are limited in scope and duration and ensures humanitarian aid continues and avoid[s] civilian casualties," the statement read.

Throughout most of the multi-sided seven-year-old civil war in Syria, Turkey and the United States jointly backed Arab fighters seeking to overthrow Assad. Since 2014, Washington has angered Turkey by growing closer to the Kurdish militia, which it supported with airstrikes, arms, training and special forces advisers on the ground to oppose Islamic State.

A protracted campaign could bring relations between the United States and its Muslim NATO ally close to a breaking point. The tensionin the relationship also encompasses disputes over visas and diplomats, the prosecution of a Turkish bank official in the U.S., and the continued residency of an exiled cleric in Pennsylvania who Erdogan blames for instigating an unsuccessful 2016 coup.

Unconfirmed gas attack in Douma

Meanwhile, Syrian activists and rescue teams say the Syrian government has launched an attack with suspected poisonous gas that has affected nearly 20 civilians in a rebel-held suburb near the capital, Damascus.

The team of first responders known as While Helmets, or Syrian Civil Defence, says the attack hit a neighbourhood in the Douma district early on Monday.

It says the rescuers evacuated more than 20 civilians, most of them women and children, from the area, which they say was hit with suspected chlorine attack. The Ghouta Media Centre, an activist-operated media, also claims the attack involved chlorine gas. Activists say a foul smell followed a series of bombings that hit Douma.

The EasternGhouta suburb of Damascus has been under intensive attack, and the UN says government forces are holding 400,000 people under siege there.

In the wake of the report, the United States urged Russia to use its influence over Assad to intervene.

U.S. State Department spokeswoman Heather Nauert said Moscow's "unwillingness or inability to restrain the Assad regime is costing innocent Syrian lives."

She said the United States has tried to hold responsible those using chemical weapons in attacks that "have killed far too many Syrians."

Such claims are not new but they are difficult to verify because of lack of chemical labs and independent testers. A UN inquiry panel had previously blamed the government for a number of chlorine and sarin attacks in Syria.

Shelling was also involved in the Ghouta attack on Monday. Syrian state media and the Syrian Observatory for Human Rights, an opposition-linked group that tracks the conflict, each said nine people were killed in the shelling.