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 "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 been forced to suspend operations and evacuate 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.