A senior Kurdish commander said Turkish forces had resumed attacks in northeastern Syria on Thursday, 48 hours after Turkey announced it would no longer use force in the region.

Turkey and Russia agreed Tuesday that the two nations' troops would patrol the Syria-Turkey border, while Kurdish forces who have been key U.S. allies in fighting the Islamic State militant group retreat from the designated "safe zone" — a strip of land along the border where Turkish president Recep Tayyip Erdogan wants to resettle refugees.

On Wednesday, President Donald Trump said the cease-fire would be permanent and announced the lifting of economic sanctions on Turkey.

But Gen. Mazloum Abdi, commander of the Syrian Democratic Forces, which controlled the area until Turkey's invasion Oct. 9, tweeted that attacks had restarted.

Turkish forces were launching "attacks on the eastern front of the Serêkaniyê," he said, using the Kurdish name for the border town of Ras al-Ayn, which has been at the center of this conflict.

In spite of the Turks announcement of the END of military operations, they and their jihadists continue to VIOLATE and launch attacks on the eastern front of the Serêkaniyê.

The guarantors of the ceasefire must carry out their responsibilities to rein in the Turks.#SDF — Mazloum Abdî مظلوم عبدي (@MazloumAbdi) October 24, 2019

Mustafa Bali, head of the SDF press office, said via Twitter that Turkish forces had been attacking the villages of Assadiya, Mishrafa and Manajer.

The "SDF will exercise its right to legitimate self defense and we are not responsible for the violation of the agreement," he wrote.

NBC News could not independently verify that hostilities had resumed.

Erdogan on Thursday confirmed an offensive against Kurdish forces if they do not withdraw from the safe zone as part of an agreement with Russia, telling his district governors "it is our natural right to crush them," Reuters reported.

Kurdish forces had moved some 18 miles from the border by Thursday, according to Russia's RIA news agency, which cited an SDF official.

Erdogan also said Turkey will build hospitals and schools in the safe zone and ensure a voluntary return of refugees, according to state TV.

Trump had paved the way for Erdogan to invade northeastern Syria and faced widespread criticism at home and abroad for not doing more to protect the Kurds, thousands of whom have fled their homes. Around 1,000 U.S. troops have withdrawn from the region, although a small number will remain.

The Syrian Democratic Forces were an ally against ISIS but Turkey considers the YPG, its biggest grouping, to be a terrorist group and a direct threat.

Meanwhile, Defense Secretary Mark Esper criticized Turkey's "unwarranted" excursion into Syria.

Speaking at the German Marshall Fund in Brussels on Thursday, he said "Turkey put us all in a very terrible situation," The Associated Press reported.