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.
The Turkish government wants to create a so-called “safe zone” in the area, where it can 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.
Dozens of civilians have been killed in the operation so far and at least 160,000 have fled the area, according to the UN. Some aid organisations have also reportedly started to evacuate their international staff over security fears.
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 a “stab in the back”.
There are fears the destabilisation could risk a resurgence of IS, as thousands of former fighters and their relatives are being detained in northern Syria. Hundreds of IS family members 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 alongside Vice-President Mike Pence in Washington DC on Monday evening.
Action was taken against Turkey’s defence and energy ministries, as well as the ministers of defence, energy and interior, the US Treasury said in a statement. The move freezes their assets in the US and bans transactions with them that involve the US financial system.
“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 Turkey – a 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 back 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.”
Vice-President Pence warned that the sanctions will worsen “unless and until Turkey embraces an immediate ceasefire” and negotiates a long-term settlement issues on the border.
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.