The US has imposed sanctions on Turkish ministries and senior government officials in response to the country’s military offensive in northern Syria.
President Donald Trump also phoned his Turkish counterpart Recep Tayyip Erdogan to demand an immediate truce, Vice-President Mike Pence said.
Mr Pence said he would travel to the region “as quickly as possible”.
The move comes after criticism of a US troop withdrawal from the region which some say gave Turkey a “green light”.
The Turkish offensive, which began last week, aims to push the Kurdish-led Syrian Democratic Forces (SDF) from the border region.
Turkey considers the biggest militia in the SDF a terrorist organisation.
Turkish forces also want to establish what their government describes as a “safe zone” in the area, to resettle up to two million Syrian refugees currently in Turkey.
Many of them are not Kurds and critics warn this could lead to ethnic cleansing of the local Kurdish population.
Kurdish-led forces have been a key ally of the US in the fight against the Islamic State (IS) group in Syria. They described the US withdrawal, which preceded Turkish action, as being a “stab in the back”.
There are fears the local destabilisation could risk a resurgence of the IS group, as thousands of former fighters and their relatives are being detained in northern Syria. Hundreds are said to have already escaped from one camp.
Facing immense pressure, Kurdish-led forces on Sunday announced a deal with the Syrian government for military support to help repel Turkey.
What are the US sanctions?
US Treasury Secretary Steven Mnuchin announced the measures, which singled out two Turkish ministries and three senior government officials, in Washington DC on Monday evening.
The US Treasury published a statement, which said that action was taken against Turkey’s defence and energy ministries, as well as the ministers of defence, energy and interior. The actions freeze their assets in the US and bans US-related transactions with them.
“The Turkish government’s actions are endangering innocent civilians, and destabilizing the region, including undermining the campaign to defeat ISIS [Islamic State],” the statement added.
President Donald Trump has faced mounting pressure to take action against the US’s Nato partner, including from Republicans usually loyal to his administration.
In a statement posted on Twitter, President Trump also said he would raise tariffs on Turkish steel to 50% and “immediately stop” negotiations related to a “$100 billion trade deal” with Turkey.
“The United States and our partners have liberated 100 percent of ISIS’s ruthless territorial caliphate,” the statement said. “Turkey must not put these gains in jeopardy.”
Appearing alongside Mr Mnuchin on Monday, Vice-President Pence warned that the sanctions “will continue and will worsen unless and until Turkey embraces an immediate ceasefire, stops the violence and agrees to negotiate a long-term settlement of the issues along the border between Turkey and Syria”.
Mr Pence said that President Trump reiterated this in Monday’s phone call with President Erdogan.
The vice-president also reiterated that the US “did not give a green light to Turkey to invade Syria”.
House Speaker Nancy Pelosi, who leads the Democrats, said on Twitter that the sanctions “fall very short of reversing the humanitarian disaster brought about by [the president’s] own erratic decision-making”.
Earlier on Monday, European Union countries committed to suspending arms exports to Turkey but stopped short of an EU-wide arms embargo.
In response, Turkey said it would examine its co-operation with the EU due to its “unlawful and biased” attitude.
What is happening in Syria?
Following the deal with Kurdish-led forces, the Syrian army began to move towards the border on Monday.
Syrian state media said government forces had entered the strategic town of Manbij, inside the area where Turkey wants to create a “safe zone”. Turkish troops and their allied Syrian militias were gathering near the town.
The deal was seen as a boost for Syrian President Bashar al-Assad as it meant his troops would return to north-eastern areas for the first time since 2012, when their withdrawal to fight rebels elsewhere allowed Kurdish militias to take control.
Despite disagreeing with their attempts at self-rule, Mr Assad did not seek to retake the territory, especially after the Kurds became partners in the coalition against IS with US troops on the ground.
Apart from fighting IS, the Kurds were fundamental for the US in limiting the influence of rivals Russia and Iran and keeping some leverage on the ground.
For now, Syrian forces will not be deployed between Tal Abyad and Ras al-Ain, where Turkey has focused its efforts. Kurdish-led officials insisted they would remain in charge politically, and retain order in the area.
The Russian government, an ally of Mr Erdogan, said it did not want to entertain the possibility of a clash between Russian and Turkish forces in Syria, and said it was in regular contact with Turkey’s authorities.
Up to 160,000 civilians had been displaced, according to UN humanitarian agency OCHA, which said the number was expected to rise.
At least 50 civilians have been killed inside Syria and another 18 over the border in southern Turkey, reports say. Kurdish forces have confirmed the deaths of 56 of their fighters while Turkey says four of its soldiers and 16 pro-Turkish Syrian fighters have been killed in Syria.
Last week, US President Donald Trump abruptly pulled dozens of US troops from pockets in the north-east of Syria after a phone call with Mr Erdogan.
The move effectively paved the way for the operation by Turkey, which views elements of the Kurdish groups in Syria as an extension of the banned Kurdistan Workers’ Party, which has fought for Kurdish autonomy in Turkey for three decades.
On Sunday the Pentagon said it was preparing to evacuate all 1,000 of its remaining troops from northern Syria, citing the “very untenable” situation on the ground and fears that Turkey would extend its attack further than originally planned.