WASHINGTON— Turkish President Recep Tayyip Erdogan and President Donald Trump met Wednesday as relations between the NATO allies are at their lowest point in decades, with Turkey drifting closer to Russia and Ankara facing a Washington backlash over its offensive against Kurds in Syria.

Erdogan and Trump have a difficult agenda. They will discuss Turkey’s decision to buy a Russian air defense system despite its membership in NATO and its incursion into neighboring Syria to attack Kurdish forces who have fought with the U.S. against the Islamic State group. The issues threaten to jeopardize a potentially lucrative trade deal between the two countries.

The leaders’ scheduled afternoon news conference, following a meeting with Republican lawmakers at the White House, will give Trump a stage to counter the first public hearings in the House impeachment inquiry. Just before warmly welcoming Erdogan to the White House, Trump tweeted that the House Democrats were “trying to stop me, because I’m fighting for you. And I’ll never let that happen.”

Trump notes Turkey has been a critical U.S. ally for decades and cites the economic upside to the relationship as a reason to overcome the differences. Some lawmakers say Erdogan should never have been invited to the White House in the first place.

Last month, the House overwhelmingly passed a bill to sanction senior Turkish officials and its army for the military incursion into Syria to fight the Kurds. Erdogan sees Kurdish forces in Syria as an extension of a separatist Kurdish group that’s been fighting inside Turkey since the 1980s.

“This is not the time or place to be extending hospitality and exchanging niceties with a dictator,” said Sen. Jeanne Shaheen, a New Hampshire Democrat who sits on the Foreign Relations and Armed Services committees.

In the Senate, two Democrats introduced legislation denouncing Turkey’s targeting of journalists, political opponents, dissidents, minorities and others. They said the Turkish government had imprisoned more than 80,000 Turkish citizens, closed more than 1,500 non-governmental organizations on terrorism-related grounds and dismissed or suspended more than 130,000 civil servants from their jobs.

In October, Trump moved U.S. troops in Syria out of the way of invading Turkish troops, a decision that critics said amounted to abandoning America’s Kurdish allies to be attacked.

“It has upended what was an oasis of stability, damaged U.S. credibility and standing on the world stage and strengthened the hands of Russia, Iran” and the Syrian government of Bashar Assad, Shaheen said.

Trump administration officials have said the president told Turkey not to invade Syria. But when Erdogan insisted, they say, Trump decided to move 28 Green Berets operating on the Turkey-Syria border so they wouldn’t be caught in a crossfire between Turkish-backed forces and the Kurds.

A State Department official said Trump is not rewarding Erdogan with a White House visit but is conducting diplomacy. The official said high-level consultations are needed because of the volatile situation in Syria that has displaced tens of thousands of people.

Amnesty International recently released a report documenting killings, human rights violations and possible war crimes caused by Turkey-backed forces in northern Syria.

“There has been a callous disregard for civilian lives, including attacks on residential areas,” said Margaret Huang, executive director of Amnesty International USA. “Over 100,000 people have fled this offensive and there are fears that the displaced are not getting access to food, to clear water, or to medical supplies.”

She said Trump must send a message to Erdogan that these actions and unlawful behavior must stop and that those responsible be held accountable.

A senior State Department official said that the U.S. is following up on reports of human rights violations and indiscriminate killings. The official was not authorized to discuss the issue publicly and spoke only on the condition of anonymity.

The U.N. High Commissioner for Human Rights has urged Turkey to investigate reported cases of summary executions committed by a Turkish-backed armed group in northern Syria. The U.N. cited video footage showing fighters with the Ahrar al-Sharqiya armed group filming themselves capturing and executing three Kurdish captives on a highway in northern Syria.

The State Department has looked into these killings and has asked Turkey to investigate. The Turks have told the U.S. that the Syrians have set up a commission, the official said, but it’s unclear what, if any, action the panel will take.

Turkey reached truce agreements with Russia and the United States last month that halted the incursion and forced Kurdish fighters to retreat from Turkey’s southern border. But Erdogan claims the Kurds have not vacated border areas and says he will give Trump a list of attacks carried out by Mazloum Abdi, the commander of the U.S.-backed Syrian Kurdish-led force.

On the U.S. side, Trump will be expressing continued concern about Erdogan’s purchase of the Russian S-400 air defense system. The U.S. and fellow NATO nations say the S-400 would aid Russian intelligence and compromise a U.S.-led fighter jet program.

The U.S. has since kicked Erdogan out of a multinational program producing components of America’s high-tech F-35 fighter jet. In response, Erdogan attended an annual Russian air show this summer in Moscow and expressed interest in buying the latest Russian Su-35 fighter jets.

Trump has not yet decided whether to impose congressional sanctions on Turkey for the S-400 purchase.