Welcoming Angela Merkel to the White House for a final time, President Joe Biden renewed his concerns to the German chancellor on Thursday about a major, nearly complete Russia-to-Germany gas pipeline but said they agreed Russia must not be allowed to use energy as a weapon.

The two discussed — though made no apparent headway — on differences over the Nord Stream 2 pipeline during a largely friendly farewell visit for Merkel as she nears the end of a political career that has spanned four American presidencies.

“On a personal note, I must tell you I will miss seeing you at our summits,” Biden said as he stood by Merkel, the second-longest serving chancellor in Germany’s history, at a late afternoon White House press conference. “I truly will.”

Merkel, who had a famously difficult relationship with former President Donald Trump, showed her ease and familiarity with Biden, who has long been a fixture in international politics, repeatedly referring to him as “Dear Joe.”

Asked to compare her relationship with Biden to hers with Trump, Merkel remained diplomatic, saying only that it was in any German chancellor’s interest to “work with every American president.” She added with a smile, “Today was a very friendly exchange.”

But their personal warmth notwithstanding, the U.S.-German relationship is entering new territory as Merkel, who is not seeking another term in September elections, nears her departure from office. There are concerns on both sides about how the two nations will negotiate growing disagreements.

The United States has long argued that the Nord Stream 2 project will threaten European energy security by increasing the continent’s reliance on Russian gas and allowing Russia to exert political pressure on vulnerable Eastern and Central European nations, particularly Ukraine. But Biden recently waived sanctions against German entities involved in the project, a move that angered many in Congress.

Merkel sought to downplay the differences and to stress that the pipeline was in addition to — not meant to displace — Ukrainian pipelines.

Merkel also raised concerns about COVID-19 travel restrictions that prevent most Europeans from traveling to the U.S.

Biden said he had brought in the head of his coronavirus task force to discuss the issue and that he expected to be able to offer a more definitive answer “within the next seven days” about when the restrictions might be eased.

While there are points of tension, Biden seemed eager to offer Merkel a proper farewell.

He hosted Merkel and her husband, as well as an array of lawmakers and administration officials, current and past, at the White House for a dinner Thursday evening. The guest list included Secretary of State Antony Blinken, as well as two of his predecessors — Hillary Clinton and Colin Powell.

Also Thursday, Merkel received an honorary doctorate, her 18th, from Johns Hopkins University and spoke at the university’s School of Advanced International Studies.