US President Donald Trump's decision to withdraw 2,000 US troops from northern Syria has dismayed many of Washington's allies, and critics say Turkey will look to benefit in its fight against US-backed Kurdish forces inside Syria.

Turkey has said any offensive targeting the Syrian Kurdish People's Protection Units (YPG) militia and the Islamic State (IS) group will be launched in the coming months.

Turkey views the YPG as a "terrorist offshoot" of the outlawed Kurdistan Workers' Party (PKK), which has waged an insurgency against the Turkish state since 1984.

But the militia has also been a key US ally in the fight against Islamic State in Syria, working with American forces on the ground there.

"Just as we did not leave our Syrian Arabs to Daesh (IS), we will not leave Syrian Kurds to the cruelty of the PKK," Erdogan said during a speech in Ankara.

A Turkish military convoy arrived overnight on Monday at the border with Syria, with local media reporting that some vehicles had entered Syria.

Erdogan's spokesman Ibrahim Kalin told reporters on Monday that a US military delegation would arrive this week to "discuss how to coordinate (the withdrawal) with their counterparts".

Kalin also said that a Turkish foreign ministry delegation would go to Washington for talks on the pull-out on January 8.

In a telephone conversation Sunday between Trump and Erdogan, which both sides described as "productive", they agreed to avoid a power vacuum in Syria after the US withdrawal.

- 'No slowdown' in IS fight -

Unlike several other allies of the United States, Turkey has praised Trump's decision to withdraw 2,000 of his ground forces from Syria.

Trump stunned the US political establishment and allies last week with his decision, days after Erdogan had warned that Ankara would soon launch an offensive in northern Syria.

Critics of Trump's decision fear that thousands of Islamic State (IS) group extremist members are still thought to be in Syria, despite Trump's claim of having defeated IS.

The US leader tweeted that Erdogan had told him Ankara would "eradicate" the last IS elements.

And Kalin vowed that there was "no question of a step backwards, vulnerability or a slowdown in the fight against Daesh (IS)".

He added: "Turkey will show the same determination against Daesh. To beat Daesh, we don't need the PKK or the YPG. We can bring peace to this region."

- Convoy enters Syria -

The Turkish military convoy with howitzers, artillery batteries and several units of the armed forces, was deployed to the border district of Elbeyli in Kilis province, state news agency Anadolu reported on Monday.

Parts of the convoy had entered Syria, the private IHA news agency reported, which said the reinforcements would take place "gradually".

The deployment began over the weekend with around 100 vehicles, the Hurriyet daily said, and crossed into the Al-Bab region, headed towards Jarabulus and YPG-held Manbij.

Jarabulus and Al-Bab were areas captured from IS during Ankara's first military operation in August 2016 which lasted until March 2017.

Military reinforcements had also been sent to the Akcakale border town and Ceylanpinar district, both in the southeastern Sanliurfa province.

Turkey conducted a second offensive with Syrian rebels against the YPG in its northwestern enclave of Afrin in January this year.

Turkish officials have previously said Ankara has no territorial ambitions in Syria.

- Warming Turkey-US relations -

Trump's move has sparked dismay among allies and turmoil within the administration. First Defence Secretary Jim Mattis resigned, then Brett McGurk, the special envoy to the anti-IS coalition, stepped down.

American support for the YPG, under the banner of the Syrian Democratic Forces (SDF) alliance, has long been a source of tension between NATO allies the US and Turkey.

But relations between the Turkish and American leaders appear to have improved substantially since a crisis in the summer over the detention of a US pastor, since released.

Last week, the US approved the sale of $3.5 billion in missiles to Turkey, after American outrage over Ankara's major arms purchase from Russia.

But there are still strains over the US refusal to extradite Fethullah Gulen, the Pennsylvania-based Muslim preacher who Turkey says ordered the 2016 failed coup.