Turkish President Tayyip Erdogan told reporters after Friday prayers in Istanbul that Turkey will continue its offensive into northeast Syria more rapidly than before if an agreement with the United States to pause the operation and allow the withdrawal of Kurdish forces is not fully implemented.

He confirmed that he had informed US President Donald Trump about the offensive in a phone call a day before launching it, The US and Turkish officials would be in constant contact to implement the agreement together.

On Friday morning, shelling was heard at the Syrian-Turkish border despite the five-day ceasefire to clear a planned "safe zone" of Kurdish militia, while the United States said the deal covered only part of the territory Ankara aims to seize.

However, Erdogan said the "safe zone" would stretch 440 km (273 miles) along Turkey's border with Syria, with its eastern edge on its border with Iraq, and added that a depth of 20 miles (32 km) was agreed on with the United States.

Erdogan added that there were no ongoing clashes in northeastern Syria, where Turkey has been carrying out an offensive, after Ankara agreed with Washington to pause its offensive while Kurdish forces withdraw.

He also told journalists that it was not a problem for Turkey if Syrian government forces, backed by Russia, were to enter areas cleared of the Syrian Kurdish YPG militia, adding that Ankara had no intention to stay in areas under its control in northern Syria.

He added that "what is necessary will be done when the time is right" about a letter from Trump in which he told Erdogan to not be a "fool" and "tough guy".