U.S. President Donald Trump on Monday raised steel tariffs on Turkey and announced sanctions over its incursion into Syria as Trump scrambled to limit the damage from his much-criticized decision to clear U.S. troops from Turkey's path.

With lawmakers in the U.S. Congress moving to impose sanctions of their own, Trump issued an executive order authorizing sanctions against current and former officials of the Turkish government for contributing to Turkey's military operation in northern Syria.

In a statement, Trump said he had increased tariffs on imports of Turkish steel back up to 50 per cent, six months after they were reduced, and would immediately stop negotiations on what he called a $100-billion trade deal with Turkey.

"Unfortunately, Turkey does not appear to be mitigating the humanitarian effects of its invasion," said Trump.

Trump also said that U.S. troops coming out of Syria will redeploy and remain in the region to monitor the situation.

Statement from President Donald J. Trump Regarding Turkey’s Actions in Northeast Syria <a href="https://t.co/ZCQC7nzmME">pic.twitter.com/ZCQC7nzmME</a> —@realDonaldTrump

The U.S. also called on Turkey to stop the invasion, and Trump is sending Vice-President Mike Pence to the region in an attempt to begin negotiations. Pence said Trump spoke directly to Turkish leader Recep Tayyip Erdogan.

"President Trump communicated to him very clearly that the United States of America wants Turkey to stop the invasion, implement an immediate ceasefire and to begin to negotiate with Kurdish forces in Syria to bring an end to the violence," Pence said.

House Speaker Nancy Pelosi, meanwhile, said Trump's package of sanctions "falls very short of reversing that humanitarian disaster."

"President Trump gave Turkey the green light to attack our Kurdish partners, betraying the thousands of fighters who gave their lives to keep the world safe from the barbarism of ISIS. His erratic decision-making is threatening lives, risking regional security and undermining America's credibility in the world," Pelosi said in a statement.

Backlash within the U.S. against President Donald Trump’s decision to pull its troops from northern Syria continues to grow. 2:15

Syrian forces welcomed into territory, state TV shows

Earlier, Russian-backed Syrian forces deployed deep inside Kurdish-held territory south of the Turkish frontier. Washington's former Kurdish allies said they had brought in the Syrian troops as an "emergency measure" to help fend off an assault by Turkey, launched last week with "a green light" from U.S. President Donald Trump that the Kurds describe as a betrayal.

The Syrian government began deploying on Monday in a major victory for President Bashar al-Assad and his principal ally Russia, which gained a military foothold across the biggest swathe of the country that had been beyond grasp.

Syrian government troops moved into towns and villages in northern Syria, including Tel Tamer, setting up a potential clash with Turkish-led forces advancing in the area. (Baderkhan Ahmad/The Associated Press)

U.S. Defence Secretary Mark Esper said he will meet with NATO allies next week to press them to take "diplomatic and economic measures" in response to Turkey's incursion. In a statement, Esper said Turkey's military action "was unnecessary and impulsive" and could result in the resurgence of ISIS.

Syrian state media reported that the army entered the Kurdish-held town of Manbij, hours after Turkey-backed opposition fighters announced that they are advancing on the city. SANA news agency gave no further details.

Monday's move was expected, coming a day after Syrian Kurdish militias struck an alliance with government forces to help fend off the Turkish offensive. Manbij houses U.S. troops, and a U.S. official said troops are still in the flashpoint city, preparing to leave.

State TV also showed residents welcoming Syrian forces into the town of Ayn Issa, which lies on another part of the highway, hundreds of kilometres away. A Syrian Democratic Forces (SDF) media official said he could not confirm these deployments.

In this photo released by the Syrian official news agency SANA, people welcome Syrian troops as they enter the town of Ayn Issa, north of Raqqa, on Monday. (SANA/The Associated Press)

Ayn Issa commands the northern approaches to Raqqa, former capital of the ISIS self-proclaimed caliphate, which Kurdish fighters recaptured from militants two years ago in one of the biggest victories of a U.S.-led campaign.

Trump's policy reversal allowed Turkey to launch a cross-border assault last week that sent tens of thousands of civilians fleeing and the Kurds scrambling to find new friends.

"After the Americans abandoned the region and gave the green light for the Turkish attack, we were forced to explore another option," senior Kurdish official Badran Jia Kurd said.

Jia Kurd described the arrangement with Assad's forces as a "preliminary military agreement," and said political aspects would be discussed later. The Kurds have led an autonomous administration in the area they controlled.

Assad aims to restore his government's authority across all of Syria after more than eight years of civil war.

Dire situation worsening, UN says

According to United Nations spokesperson Stephane Dujarric, at least 160,000 civilians have been displaced since the Turkish offensive began on Oct. 9, mostly from violence around the towns of Tal Abyad and Ras al-Ayn.

Dujarric told reporters that the UN World Food Program has so far provided immediate food assistance to more than 70,000 people fleeing towns as the fighting continues.

"Most of the displaced are staying with relatives or host communities, but increasing numbers are arriving at collective shelters in the area," he said.

Russian-backed Syrian forces pushed into Kurdish-held territory after U.S. troops withdrew — setting up a potential clash with Turkey’s forces and threatening the lives of those caught in the middle. 1:39

Northeast Syria was already facing a humanitarian crisis before the Turkish offensive, with 1.8 million of the three million women, children and men in the region in need of assistance, "including over 910,000 in acute need," Dujarric said.

He said there are also "heightened concerns" for vulnerable people in camps for the displaced, including al-Hol. That camp holds some 68,000 people who fled the last battlefields of ISIS — 94 per cent of them women and children.

Turkey, however, justified its ongoing invasion of northeast Syria to the UN by saying it's exercising its right to self-defence under the UN Charter, according to a letter circulated Monday.

Ankara said the military offensive was undertaken to counter an "imminent terrorist threat" and to ensure the security of its borders from ISIS and Syrian Kurdish militias, whom it calls "terrorists."

Turkey's UN Ambassador Feridun Sinirlioglu said in the letter to the Security Council dated Oct. 9 that its counter-terrorism operation will be "proportionate, measured and responsible."

"The operation will target only terrorists and their hideouts, shelters, emplacements, weapons, vehicles and equipment," he said. "All precautions are taken to avoid collateral damage to the civilian population."

Smoke rises above the Syrian border town of Tel Abyad, as seen from Akcakale, Turkey, on Sunday. (Stoyan Nenov/Reuters)

The Turkish assault has raised Western concerns that the Kurds, holding large swathes of northern Syria previously controlled by ISIS, would be unable to keep thousands of jihadists in jail and tens of thousands of their family members in camps.

The region's Kurdish-led administration said 785 ISIS-affiliated foreigners escaped a camp at Ayn Issa over the weekend. The British-based war monitor Syrian Observatory for Human Rights, citing sources in the camp, said around 100 people had escaped.

Trump said Monday that Kurdish-led forces in northeast Syria may be releasing captive ISIS militants to lure U.S. troops back to the area, adding they could be easily recaptured.

International condemnation

The EU unanimously condemned Turkey's military move into northern Syria and asked all member states to stop selling arms to the nation.

Spanish Foreign Minister Josep Borrell said the 28 member states "have unanimously decided to condemn — that is the verb, not concern, not worry — but to condemn in strong terms what in the end is a military attack."

Borrell said the EU called on Turkey "to stop immediately these military actions." He said that at the meeting of EU foreign ministers, the member states "call also [on] all member states to stop selling any kind of arms" to Turkey.

Boys wave Turkish flags as a military convoy drives near the border town of Akcakale in the Turkish province of Sanliurfa near the border of Syria on Monday. Turkey has pressed on with its offensive into northern Syria. (Kemal Aslan/Reuters)

UN Secretary General António Guterres on Monday called for "immediate de-escalation" and urged all parties to resolve their differences through peaceful means.

Iran's president also called on Turkey to halt its military offensive. "We do not accept the method that they have chosen," Hassan Rouhani said at a press conference on Monday in his first direct comments on the offensive.

Iran and Russia have allied with the Syrian government in the country's eight-year war.