President Trump expressed optimism that the United States and Turkey are on their way to resolving the many differences between them, but he provided few details at a news conference with Turkish President Recep Tayyip Erdogan after a day of White House meetings.

Trump said a tentative cease-fire is holding in northeastern Syria and thanked Erdogan for “his cooperation.” He said the two leaders “made tremendous progress” toward more than quadrupling bilateral trade — to $100 billion — and “hopefully will be able to resolve” a conflict over Turkey’s purchase of a sophisticated Russian missile defense system.

Calling Turkey a “great NATO ally and a strategic partner of the United States around the world,” Trump described their talks as “wonderful and productive.”

He said he was paying no attention to House hearings in the impeachment inquiry over Ukraine that was taking place at the other end of Pennsylvania Avenue. “I hear it’s a joke,” he said. “I haven’t watched for one minute, because I’ve been with the president, which is much more important as far as I’m concerned.”

The most immediate tensions surrounding Erdogan’s visit came with Turkey’s military offensive against Kurdish fighters in neighboring Syria, begun last month after Trump announced he would withdraw U.S. troops from the area. The withdrawal engendered sharp bipartisan criticism on Capitol Hill, where lawmakers of both parties accused Trump of abandoning the U.S.-allied Kurds, who had borne the brunt of the fight against the Islamic State in Syria.

But Trump rebuffed lawmakers who had called on him to cancel the invitation, saying Erdogan has lived up to an Oct. 17 agreement negotiated with Vice President Pence to limit Turkey’s incursion into Syria and allow it to create a long-coveted buffer zone at least 20 miles deep inside Syria.

Widespread reports of war crimes committed by Turkish-controlled Syrian militias that are part of the invasion force are being investigated by the Pentagon. At the same time, Turkey’s military operations have displaced an estimated 100,000 people from their homes in northern Syria, according to the United Nations.

Erdogan shared Trump’s assessment of their talks as productive, but he also took the news conference as an opportunity to list outstanding grievances. Chief among them is the U.S. failure to extradite Turkish cleric Fethullah Gulen, a permanent U.S. resident, who Erdogan’s government claims masterminded a 2016 coup attempt.

So far, the Justice Department has deemed evidence Turkey has submitted for Gulen’s extradition to be insufficient. “We are not getting the best of news out of the United States,” Erdogan said. But he had brought additional information with him, he said, and “in light of these documents, I think they will appreciate the situation.”

Erdogan also criticized the House’s passage last month of a resolution officially designating the 1915 Turkish slaughter of an estimated 1.5 million Armenians as genocide, something Turkey had strenuously lobbied against for years.

At one point during Wednesday’s talks, Trump invited a group of Republican senators critical of Turkey to join them.

“We’re having a very good discussion,” Trump told reporters as Sens. Lindsey O. Graham (S.C.), Ted Cruz (Tex.), James E. Risch (Idaho), Rick Scott (Fla.) and Joni Ernst (Iowa) sat on couches before him and Erdogan in the Oval Office.

“The purpose of this meeting is to have an American civics lesson with our Turkish friends,” said Graham, who last month called on Trump to “stand up to Erdogan” and branded the Turkish leader as a “thug.”

In a briefing Tuesday for reporters, senior administration officials stressed the importance of preserving Turkey as a NATO and regional ally, no matter their differences, adding that Trump “believes in working the hardest on the hardest issues.”

At the news conference, Trump said “the U.S.-Turkey alliance can be a powerful alliance for security and stability, not only in the Middle East, but beyond.”

That alliance has been sorely tested in recent years. In Syria, Turkey considers the Kurdish fighters allied with the United States to be terrorists who threaten Turkey’s security.

Erdogan recounted what he said were both Turkish and Syrian Kurdish attacks against the Turkish military. “But some circles who are empathetic towards these terrorist organizations are feeling deeply upset,” he said in a clear reference to congressional critics. “They are deeply disturbed, and they are using this information in order to cloud the understanding of the public opinion and that — that perception with the eventual gain or goal of harming our relations.”

Sanctions on Turkey were mandated under U.S. law when it purchased Russia’s S-400 missile defense system, although Trump has delayed imposing them. The purchase came after Turkey and the United States failed to reach an agreement over the price and configuration of the U.S.-made Patriot missile defense system. After the Russian weapons were delivered earlier this year, the administration announced it was cutting Turkey’s participation in the U.S.-led consortium manufacturing components for the F-35 combat jet — a major blow to the Turkish defense industry — and canceled the Turkish purchase of more than 100 of the planes.

Trump said Wednesday that the S-400 issue “creates some very serious challenges for us” but that “we’ve asked our secretary of state and ministry of foreign affairs and our respective national security advisers to immediately work on resolving” the problem. He provided no details, but senior administration officials have said that one possible solution may be that Turkey will not unpack or deploy the Russian system.

At a House hearing Wednesday, Defense Department, military and industry officials testified that Turkey is still providing parts for the F-35, despite its announced suspension from the program, although alternative providers have been located to substitute for Turkish components.

Senior administration officials said Trump had offered Erdogan a White House visit, a workaround for the S-400 program and a trade deal when he spoke to the Turkish leader on Oct. 6 in a failed attempt to prevent the military incursion three days before it launched. In a letter to Erdogan the day after Turkish troops crossed the Syrian border, Trump told him it should stop, offering to “do a deal” and cautioning him: “Don’t be a fool!”

Turkish officials said the letter was disrespectful and that Erdogan had tossed it “in the bin.” But Erdogan said Wednesday that he had brought the Oct. 9 missive to Washington with him and given it back to Trump.

After initially withdrawing a few dozen U.S. troops from the border area to avoid a clash with the advancing Turks, Trump said he was going to pull the entire 1,000-troop force out of Syria. Graham, a Trump confidant, initially called that “the biggest mistake of his presidency.” But he later congratulated the president on the cease-fire deal. Trump subsequently agreed to leave about 600 troops in northeastern Syria.

In advance of their meeting with Erdogan, Ernst and Scott said that they planned to confront him on the S-400 purchase.

“I want to make sure he understands that we’re going to have to implement sanctions,” said Scott, a vocal critic of Erdogan. “The president doesn’t have a choice. And that’s not going to be good for Turkey.”

Last month, an overwhelming House majority voted to impose separate sanctions on Erdogan’s regime for its assault on Syria.

Risch, chairman of the Senate Foreign Relations Committee, and his Democratic counterpart, Sen. Robert Menendez (N.J.), have proposed a similar sanctions package, as have Graham and Sen. Chris Van Hollen (D-Md.), but neither bill has been scheduled for a floor vote.

Several Democratic senators joined Van Hollen in calling Trump’s decision to host Erdogan at the White House as “absolutely shameful,” and some Republicans voiced similar, if more muted, concerns.

“This is an unfortunate time, in my opinion, for the visit,” Sen. Mitt Romney (R-Utah), the party’s 2012 presidential nominee, said in an interview Wednesday. “I would like Turkey to adopt appropriate human rights and democratic principles, to reject the purchase of the Russian military system and to be a more reliable member of NATO.”

In a statement issued Wednesday morning, Senate Majority Leader Mitch McConnell (R-Ky.) said he “shared my colleagues’ uneasiness at seeing President Erdogan honored at the White House.”

But, he said, “I urge this body to remain clear-eyed about our nation’s vital interests in the Middle East and the fact that advancing them will mean strengthening our relationship with this NATO ally, not weakening it further.”