American servicemen were killed in a suicide attack that targeted the northern Syrian city of Manbij on Wednesday, the US-led coalition said.

It did not specify how many soldiers were killed in the blast, which was claimed by ISIS, but Turkish President Tayyip Erdogan said 20 people were killed in the incident, including five US troops.

The US servicemen were conducting a "routine patrol" in Manbij at the time of the attack, the coalition said in a statement on Twitter.

U.S. service members were killed during an explosion while conducting a routine patrol in Syria today. We are still gathering information and will share additional details at a later time. — OIR Spokesperson (@OIRSpox) January 16, 2019

The White House said President Donald Trump has been fully briefed.

"We will continue to monitor the ongoing situation in Syria," Press Secretary Sarah Sanders told reporters during a briefing.

Meanwhile, Vice President Mike Pence told a gathering in Washington of US ambassadors that, "ISIS has been defeated", without referencing the attack earlier on Wednesday.

The UK-based Syrian Observatory for Human Rights said at least 16 people had died, including five US-backed fighters as well as nine civilians.

The war monitor reported that at least two US troops died in the blast, which it says targeted a restaurant.

The Kurdish-run Hawar news agency said three American servicemen, two US-backed local fighters and 13 civillians were killed in the incident.

Amaq – ISIS's media wing – claimed that the attacker used a suicide vest to strike at coalition forces near a restaurant in the flashpoint Kurdish-held city.

A video posted on social media networks purported to show the moment the attacker detonated his suicide vest.

The National could not independently verify the authenticity of the footage.

Minbicteki patlama bir intihar saldırısıdır. pic.twitter.com/zGJUWVbABD — Deniz Fırtına (@DenizFrtna14) January 16, 2019

The attack came one day after coalition forces and US-backed fighters captured a key village from ISIS in their last holdout east of the Euphrates River, leaving just 15 sq kms of land under the militants' control.

It is the first major attack that explicitly targets US forces in Syria since Mr Trump announced that he would be withdrawing troops from the country.

Mr Erdogan on Wednesday said he did not believe the attack would impact Mr Trump's decision to withdraw from Syria.

He said the American president won’t halt the withdrawal because it would mean ISIS wins.

Sharfan Darwish, a spokesman for the Kurdish-led Manbij Military Council (MMC), said the blast took place on a crowded street.

He said there were casualties but did not specify how many people died.

داعش، مسئولیت حمله انتحاری امروز به منبج را برعهده گرفت. طی این حمله تاکنون ۸ سرباز آمریکایی کشته و ۱۷ سرباز مجروح شدند pic.twitter.com/7KLflPpWUl — Reza Kiani (@rkeiani) January 16, 2019

The Observatory said a helicopter it claimed belonged to the coalition took troops to a hospital east of the Euphrates River.

A video posted by reporters and activists purported to show the aircraft landing in Manbij after the attack.

#حسين_مرتضى مشاهد تُظهر هبوط مروحية اميركية لنقل القتلى والجرحى من الجنود الامركيين جراء الانفجار في منبج بريف حلب pic.twitter.com/gm3Ffq5sXP — #حسين_مرتضى (@HoseinMortada) January 16, 2019

Videos and pictures posted on social media by Hawar also showed damaged shopfronts and shattered windows.

The US-backed Manbij Military Council, which is linked the Kurdish People’s Protection Units (YPG), has controlled Manbij since they seized the city from ISIS in 2016.

The US-led coalition conducts regular patrols inside Manbij and on the outskirts of the city.

Russia’s military police recently deployed to areas outside Manbij, where they are conducting patrols in a zone adjacent to rebel-held territory in Aleppo province.

Wednesday's incident was not the first time that forces of the US-led coalition were subjected to attacks in the area, although they have been rare.

In March last year, a roadside bomb killed two coalition personnel, an American and a Briton, and wounded five in Manbij.

The latest attack comes as US forces prepare to withdraw from the war-torn country following a shock announcement by Mr Trump last month.

Turkey has repeatedly threatened to attack Kurdish forces in Manbij, stirring tensions with the American President, who has warned Ankara against such an offensive.

Separately, the US-backed Syrian Democratic Forces (SDF) said it would support efforts to establish a safe zone in northern Syria, one day after Turkey's president said his country would establish such a zone.

"We will offer all the support and assistance to set up the safe zone that is being discussed, in a way that guarantees the protection of all co-existing sects and ethnicities from annihilation," the Kurdish-led alliance said in a statement.