ANKARA (Reuters) - President Tayyip Erdogan told U.S. President Donald Trump that Turkey will never declare a ceasefire in northeastern Syria and that it will not negotiate with Kurdish forces it is fighting in its offensive into the region.

Turkey pressed ahead with its offensive against the Syrian Kurdish YPG militia in northern Syria on Tuesday despite U.S. sanctions and calls for it to stop, while Syria’s Russia-backed army moved on the key city of Manbij that was abandoned by U.S. forces.

The YPG, the key component of the forces who fought Islamic State, is seen by Ankara as a terrorist group linked to Kurdish separatist insurgents in Turkey.

On Monday, Trump announced sanctions on Turkey to punish it for the offensive. On Tuesday, a senior U.S. official said Washington would threaten more sanctions to persuade Turkey to reach a ceasefire and halt its offensive.

However, speaking to reporters on a flight back from Baku, Erdogan said the offensive would continue until it reaches its aims, and added that he was not worried about sanctions.

“They say ‘declare a ceasefire’. We will never declare a ceasefire,” Erdogan said. “They are pressuring us to stop the operation. They are announcing sanctions. Our goal is clear. We are not worried about any sanctions,” he said.

Erdogan said he told Trump in a phone call earlier this week that he should send a U.S. delegation to Ankara to discuss their demands and try to reach an agreement. The White House said on Tuesday that Vice President Mike Pence will meet with Erdogan in Ankara on Thursday.

Trump’s decision to withdraw U.S. troops from northern Syria after a phone call with Erdogan not only cleared the way for the Turkish incursion, but it also gives a free hand to Washington’s adversaries in the world’s deadliest ongoing war, namely Syrian President Bashar al-Assad and his Russian and Iranian allies.

Syrian army deployments into Kurdish-held territory mark a victory for Assad and Russia, giving them a foothold in the biggest remaining swathe of Syria that had been beyond their grasp through much of its eight-year-old war.

Asked about the deployment of Syrian forces to the northern town of Manbij, Erdogan, who has backed Syrian rebels fighting to oust Assad, said he was not bothered.

“The regime entering Manbij is not very negative for me. Why? It’s their lands after all. But, what is important to me is that the terrorist organization does not remain there,” Erdogan said, referring to the YPG.

“I told this to Mr Putin as well. If you are clearing Manbij of terrorist organizations, then go ahead, you or the regime can provide all the logistics. But if you are not going to do this, the people there are telling us to save them,” he added.

Erdogan also said that an attack from Manbij on Tuesday, which killed one Turkish soldier, was launched by the Syrian government, and that Turkey “made the regime pay a heavy price” in retaliation.

Separately, the Turkish presidency said late on Tuesday that Erdogan told Russian President Vladimir Putin in a phone call that Turkey’s operation would contribute to counter-terrorism efforts, Syria’s territorial integrity, and a political solution process.