NEWS BRIEF Hours before U.S. Vice President Joe Biden landed in Turkey on Wednesday, the Turkish military drove tanks across the Syrian border, protected overhead by U.S. aircraft, in what is Turkey’s largest ground operation in Syria.

Turkish forces are aiming to take control of the city of Jarabulus from the Islamic State, which is its last stronghold on the Turkey border. They also plan to drive out Syrian Kurdish rebels in the area, which the U.S. supports and depends on in its military campaign against ISIS, but which Turkey considers a threat.

In a press conference in Ankara after he arrived, Biden said the U.S. is prepared to drop its support of the Syrian Kurds unless they pulled back from Jarabulus, beyond the Euphrates River, which runs to the east of the town. The Kurds, Biden reassured the Turkish government, “cannot—will not—under any circumstance get American support if they do not keep that commitment.”

The U.S. support of Kurdish rebels has caused tension between the U.S. and Turkey in the last year. The Kurdish rebels have been instrumental in helping the U.S.-led coalition against ISIS in Syria. But the Turkish government believes they are connected to groups in Turkey’s southeast that it accuses of leading a separatist insurgency and carrying out terrorists attacks. Biden’s announcement, combined with the U.S. air support provided to Turkish forces in the new operation, is an indication the two countries are attempting to ease their strained relations.

Jarabulus has served as ISIS’s main smuggling route, and ISIS uses the town to control illicit trade in the area, and to ferry in foreign fighters hoping to join its ranks. The Turkish advance across the Syrian border and toward the city began around 4 a.m. local time, as Reuters reported:

The Turkish army began firing artillery rounds into Jarablus at around 0100 GMT and Turkish and U.S. warplanes pounded Islamic State targets with air strikes.

It was the first time warplanes from Turkey have struck in Syria since November, when Turkey downed a Russian warplane near the border, and the first significant incursion by Turkish special forces since a brief operation to relocate the tomb of Suleyman Shah, a revered Ottoman figure, in February 2015.

The offensive comes days after a suicide bomber attacked a Kurdish wedding in Turkey. The blast killed 54 people, and was widely blamed on ISIS. After that, Turkish officials said they would target terrorists along the country’s border.

U.S. relations with Turkey have sunk to their lowest level in decades. Last month’s attempted coup to remove Turkey’s president, Recep Tayyip Erdoğan, from power, only worsened matters. Erdogan and his supporters have blamed the coup on a former imam and political leader, Fethullah Gulen, who lives in self-imposed exile in Pennsylvania. Erdogan has demanded the U.S. extradite Gulen, but the U.S. has not said if it will do so, and there are still doubts as to who was actually behind the coup.

The conflict between Turkey and the Kurds goes back decades. Turkey recently has blamed Kurdish groups, some of whom it has linked to rebels supported by the U.S., for terrorist bombings in the country.