The US military's hands-off approach on the ground in Syria is ceding influence to Russia and Iran, a top lawmaker warned Tuesday, as the Pentagon said it has seen the Islamic State "resurge" in parts of the country.

Senate Foreign Relations Committee chairman Bob Corker said Moscow and Tehran have "significant influence" in the war-ravaged Middle Eastern nation due to their years-long commitment, while President Donald Trump signals the US could be headed for the exits there.

When asked whether he wanted a greater US troop presence to shape events in Syria, Corker was somber.

"I think the administration's plans are to complete the efforts against ISIS and to not be involved," he said, using another acronym for the jihadist group.

A frustrated Corker spoke after exiting a classified briefing by Secretary of Defense James Mattis and top generals, who explained the Pentagon's strategy to lawmakers following last weekend's missile strikes on Syria.

"Syria is Russia and Iran's now. They will be determining the future," he said.

"We may be at the table, but when you're just talking and have nothing to do with shaping what's happening on the ground, you're just talking."

Republican Senator Lindsey Graham, expressed alarm about a lack of US engagement in the country where insurgents have waged a brutal civil war against the regime of President Bashar al-Assad.

"Everything in that briefing made me more worried, not less," he said.

– 'All tweet, no action' –

"There is no military strategy on the table to deal with the malign influence of Iran and Russia," Graham said.

After the pinpoint air strikes that Trump telegraphed on social media, "I think Assad… believes we're all tweet, no action," he added.

Democrats joined in the criticism, with Senator Chris Coons, warning that Trump's administration has "failed to deliver on a coherent plan" in Syria.

"If we completely withdraw, our leverage in any diplomatic resolution or reconstruction, or any hope for a post-Assad Syria, goes away."

Meanwhile, Colonel Ryan Dillon, a spokesman for the US-led coalition fighting IS in Syria and Iraq, said Assad's regime and Russia have not always been able to hold the terrain recaptured from IS.

"As we look at ISIS in areas where we are not operating, where we are not supporting our partners on the ground, there has been ISIS elements who have been able to come back and take territory (including in) some of the neighborhoods in southern Damascus," Dillon said.

"We've seen ISIS start to resurge in areas west of the Euphrates River."

The US military is closely watching IS in Syria and Iraq, where the jihadists have lost 98 percent of the land they once held, according to the Pentagon.

Progress however has halted in recent weeks in areas where the US-led coalition is fighting IS through the Syrian Democratic Forces, a local proxy group, due to Turkish military action in the north.

Ankara in January launched a bloody operation around Afrin to push Kurdish fighters out of the city.

Many of the Kurds who had been engaged in anti-IS operations have quit that fight to support comrades in Afrin.

US plans to send American 'IS fighter' to third country
Washington (AFP) April 17, 2018 –

The US government intends to hand over to a third country an American citizen captured in Syria allegedly fighting for the Islamic State group, rather than present him to the US justice system, a court filing showed Tuesday.

In the Trump administration's first decision on how to deal with citizens caught fighting for a designated terror group, the US military plans to turn over the man, a dual US-Saudi citizen born in the United States and now held in Iraq, to an unnamed country as early as late Thursday.

The American Civil Liberties Union, which has been representing the man, known in court documents only as "John Doe", said it plans to ask the court to block the transfer, arguing that he has not been charged with a crime and has the right to due process under US laws.

"The Trump administration has been detaining this American citizen unlawfully for more than seven months, and forcibly rendering him to another country would be an unconscionable violation of his constitutional rights," said ACLU attorney Jonathan Hafetz.

"He should either be charged or freed, not handed over to an unnamed foreign government."

The notice was made in a sealed two-page filing to the federal district court in Washington DC late Monday. A heavily redacted version of the filing was released Tuesday, saying the government had bowed to the court's requirement that it give a 72 hour notification before it intends to transfer the detainee.

The country he will be transferred to was blacked out in the public document. The government has earlier said it has two countries he could be sent to; one is widely presumed to be Saudi Arabia, the second could be Iraq.

The man is the only known US citizen held as an alleged enemy combatant from the battlefields of Iraq and Syria.

On September 14 the Pentagon confirmed that they were holding him, saying he had been fighting for the Islamic State group and surrendered to the allied Syrian Democratic Forces in Syria days earlier.

He was moved to Iraq where he has been interrogated by military and FBI investigators. The International Committee of the Red Cross was given access to him, and the ACLU sued to be able to represent him.

In subsequent discussions with him, the ACLU says he asserted his habeas corpus rights to be charged under US law or be freed.

It's not clear why the government refuses to hand him over to the US justice system, as other Americans accused of terrorism have been.

But analysts think the Trump administration wants to avoid the fundamental question of whether an American caught fighting for Islamic State has any rights.

Between 100 to 200 US nationals traveled to Syria and Iraq after 2010 to work and fight in their ranks, according to various estimates.

A handful are known to have been killed, but the number isn't clear: the US has not provided any data.

Iraq and Syria's Kurds are holding a large number of captured IS "foreign fighters", including some with European nationality. Most of their home countries don't want them, posing a dilemma that US Defense Secretary Jim Mattis said in February was "an international problem."

Trump has pledged to be tough on any US jihadists, threatening to send them to the military prison at Guantanamo Bay, Cuba, where 41 non-American detainees are held.

But rights lawyers say that putting "John Doe" in Guantanamo would also violate his rights as a citizen.