US Defense Secretary Jim Mattis claims that a swift withdrawal of US troops from Syria will be a "strategic blunder" as pro-Damascus forces edge closer to victory through sweeping gains against foreign-backed Takfiri terrorists.

Speaking at a meeting of NATO defense ministers in Brussels on Friday, Mattis linked the US pullout from Syria to the fate of the UN-brokered Geneva peace talks.

"In Syria, leaving the field before the special envoy Staffan de Mistura achieves success in advancing the Geneva political process we all signed for under the UN security council resolution would be a strategic blunder, undercutting our diplomats and giving the terrorists the opportunity to recover," he said.

The Pentagon says there are roughly 2,000 US troops, mostly in northern Syria where they collaborate with Kurdish militants. French troops recently joined them amid reports that they planned to carve out a statelet outside the Syrian control.

This is while Washington and its allies have been bombarding what they call Daesh positions inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate. The strikes have on many occasions resulted in civilian casualties and failed to fulfill their declared aim of countering terrorism.

Late last month, Syria’s President Bashar al-Assad said the US “is losing its cards” as the militant groups it supports have lost significant ground, emphasizing that the Americans “should leave” Syrian soil.

He also urged the US to “to learn the lesson” from Iraq, saying, “People will not accept foreigners in this region anymore.”

In his remarks, Mattis cautioned against "leaving a vacuum" in Syria that the Damascus government and its allies could take advantage of.

"Our military campaign in Syria continues," he said. "As the operations ultimately draw to a close, we must avoid leaving a vacuum in Syria that can be exploited by the Assad regime or its supporters."

The Trump administration has suggested replacing US troops in Syria with a Saudi-led force, but the proposal is facing substantial obstacles and could potentially exacerbate the conflict.

Saudi Foreign Minister Adel al-Jubeir has said his government was talking to Washington about raising such a force.

The Wall Street Journal reported in April that the US national security adviser, John Bolton, had called the Egyptian intelligence chief Abbas Kamel to ask Cairo to play a part in building a force.

Worried about the Syrian army advances in militant-held areas, the US and Israel have recently stepped up their attacks on Syrian military positions in what is widely viewed as an attempt to prop up their sponsored terrorist groups.

Daesh raid on Bukamal

On Friday, the so-called Syrian Observatory for Human Rights (SOHR) reported that Daesh had captured several neighborhoods of the eastern Syrian city of al-Bukamal near the Iraqi border.

SOHR head Rami Abdel Rahman said at least 25 pro-Damascus forces and 18 militants had been killed in the assault.

“It’s the biggest attack on al-Bukamal since they (terrorists) lost it” in November 2017, he said. “Daesh now controls several parts of the city. Fighting is ongoing in the center."

Russia, Syrian and Iranian officials have repeatedly said that the US was assisting militants in the Arab country. SOHR, which is sympathetic to anti-Assad militants, has reported in the past on US airlifting of Daesh commanders in the areas which the Takfiri group faced imminent rout.