Turkish President Recep Tayyip Erdogan has warned that his country would restart its operation against Kurdish forces in Syria on Tuesday evening, if they do not withdraw from a "safe zone".

After US Vice President Mike Pence came to Ankara for talks with Mr Erdogan yesterday, the NATO allies agreed Turkey would suspend its offensive for five days in northern Syria while Kurdish fighters withdraw from the area.

It comes as shelling could be heard at the Syrian-Turkish border this morning despite the suspension.

"If the promises are kept until Tuesday evening, the safe zone issue will be resolved. If it fails, the operation... will start the minute 120 hours are over," Mr Erdogan told reporters during a briefing in Istanbul.

He said Turkish armed forces would remain in the region "because the security there requires this", adding that there had been no issues so far.

But the Syrian Observatory for Human Rights said there were Turkish air strikes on the village of Bab al-Kheir, east of Ras al-Ain on the border. The war monitor said five civilians were killed.

The SDF said air and artillery attacks continued to target its positions and civilian targets in Ral al Ain.

"Turkey is violating the ceasefire agreement by continuing to attack the town since last night," SDF spokesman Mustafa Bali tweeted.

Turkey launched the cross-border incursion on 9 October after repeatedly threatening to clear the border area from the Kurdish People's Protection Units (YPG) militia.

While US President Donald Trump appeared to initially green light the offensive, he made repeated threats against Turkey, often in tweets, following international outrage.

He then sent Mr Pence and the US Secretary of State Mike Pompeo with other US officials to Ankara to work out a deal, which was announced on yesterday after hours of talks.

Mr Erdogan said the "safe zone" would be 32km deep, and 444km in length, "not between Kobane and Tal Abyad".

He added that the region between the border towns of Tal Abyad and Ras al-Ain had been cleared, "but this is not over. The process is ongoing".

Just hours before the US-Turkey talks, a bizarre letter appeared in the US media from Mr Trump to Mr Erdogan, in which the US leader urged Mr Erdogan not to be a "fool" and warned his Turkish counterpart that history risked branding him a "devil".

Turkish media reported that Mr Erdogan had "binned" the letter.

The Turkish president has said the letter was not in line with "political and diplomatic courtesy ... but our mutual love and respect does not allow us to keep it on the agenda".

Kurdish authorities in northeastern Syria have accused Turkey of resorting to banned weapons such as napalm and white phosphorus munitions, which Mr Erdogan denied.

"There are certainly no chemical weapons in the inventory of our armed forces. This is all slander against our armed forces," he added.

He accused the YPG of freeing nearly 750 IS extremists, including 150 Turks, but said 195 of them had been caught.