Turkey has boosted its military positions on the border with Syria after saying it is ready for a long-threatened operation that could target Kurdish forces long allied to the US.

Dozens of military vehicles were sent to the area following a US troop withdrawal from north-eastern Syria.

President Trump defended his move again on Tuesday, saying the Kurds had not been abandoned, calling them "special".

His controversial decision was widely condemned at home and abroad.

Turkey regards the Kurdish militias, which dominate the US-allied Syrian Democratic Forces (SDF), as terrorists, and the US pullout was seen as paving the way for an offensive.

Convoys of trucks with armoured personnel carriers and tanks were seen heading to the Turkish border town of Akcakale on Tuesday night. Images of buses carrying personnel were shown by state news agency Anadolu.

Trump: Kurds 'wonderful fighters'

Mr Trump said his pullout - described as a "stab in the back" by Kurdish forces who helped defeat Islamic State (IS) in Syria - affected "only 50 soldiers" of some 1,000 US troops in the country.

In a series of tweets, Mr Trump softened his tone, praising Turkey as a trade partner and Nato ally, hours after saying he would "destroy and obliterate" its economy if the country went "off limits".

"We may be in the process of leaving Syria, but in no way have we abandoned the Kurds, who are special people and wonderful fighters," Mr Trump said, adding that the US was helping the Kurds "financially [and with] weapons".

"Any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency," the president said as he described the relationship between the two countries as "very good".

In a statement, the Pentagon said "unfortunately, Turkey has chosen to act unilaterally" and that the US personnel were removed "to ensure their safety" but that they were not being sent out of Syria.

Turkey says it wants to set up a 480km (300 mile) long and 30km deep "safe zone" along its border with Syria, to resettle up to two million of the more than 3.6 million Syrian refugees currently living in Turkey.

Meanwhile, the White House confirmed that Turkish President Recep Tayyip Erdogan would visit the US on 13 November at the invitation of President Trump.

Eyes trained on the border

BBC's Orla Guerin in Akcakale on the Turkish-Syria frontier

On both sides of the border wall there is a sense that an offensive is coming, and probably soon. There is no indication that the crescendo of international concern is altering Turkey's plans for establishing a "safe zone".

Dozens of TV cameras - local and international - are now trained on the border wall. For civilians and refugees on the other side, in north-eastern Syria, there are real fears of what a Turkish invasion could mean.

Even if it is limited in scope - and it is unclear if it will be - it could cause massive displacement. And what of the thousands of Islamic State prisoners being held in Kurdish-run detention centres?

A spokesman for the SDF told the BBC that if the attack happened, they would have to focus on defending themselves and would have to withdraw some of their forces from the jails, and from areas recently liberated from IS.

Turkey: 'Preparations complete'

Earlier, Turkey's defence ministry said "all preparations for the operation have been completed," adding that the establishment of the "safe zone" was "essential" for Syrians and for peace in the region.

Separately, Vice-President Fuat Oktay said the country would not bow to threats over its Syria plans, saying: "Where Turkey's security is concerned, we determine our own path but we set our own limits."

The comment appeared to be a response to Mr Trump's earlier threat to target the Turkish economy, reportedly an attempt by the president to placate domestic criticism, including from Republican Senate Majority Leader Mitch McConnell.

In other developments:

• SDF Gen Mazloum Abdi told the BBC it would be a historic mistake if the US withdrew its troops from Syria without the full eradication of IS
• The Kurds said talks with the Syrian government and Russia were being considered "to block the Turkish attack"
• Russia's security council said "it was important for everyone to avoid any action that could create obstacles to a peaceful settlement in Syria"
• The Turkish military carried out strikes targeting the Syrian-Iraqi border to cut-off a transit route used by Kurdish forces, according to Reuters news agency

If Turkey takes over areas controlled by Kurdish-led forces, the White House said Turkey would be responsible for all IS fighters held there.