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U.S. President Joe Biden told Ukrainian President Volodymyr Zelensky in a phone call Monday that he looks forward to welcoming him to the White House after returning from his summer trip to Europe, according to national security adviser Jake Sullivan.

In an hour-long interview with Axios on Friday, Zelensky urged Biden to meet with him face to face before a June 16 summit with Russian President Vladimir Putin — offering to join him "at any moment and at any spot on the planet," Axios reports.

The plea from Zelensky came after Biden waived sanctions on the corporate entity and CEO overseeing the construction of the Nord Stream 2 pipeline, which will allow Russia to circumvent Ukraine and deliver natural gas directly to Germany.

Ukraine views the pipeline as a dire national security threat, one that Zelensky described as "a real weapon" in Russia's hands.

Read alsoZelensky: Issue of Ukraine's accession to NATO should be resolved immediatelyThe State Department itself has said Nord Stream 2 is a "Russian geopolitical project that threatens European energy security and that of Ukraine," but Biden has signaled that he does not want to damage relations with Germany over a pipeline that will inevitably be completed anyway.

A U.S. administration source told Axios the White House was considering inviting Zelensky to Washington before Biden’s summit with Putin, but declined to move ahead with the meeting after Zelensky's decision to replace the management of state energy company Naftogaz.

That move led to concerns in the administration he was backsliding on anti-corruption efforts.

"I have come into this briefing room from the Oval Office where President Biden was on the phone with President Zelensky of Ukraine," Sullivan told reporters Monday.

"This is a call they had been planning to make in advance of President Biden going to Europe and meeting with President Putin," he continued.

"They had the opportunity to talk at some length about all of the issues in the U.S.-Ukraine relationship and President Biden was able to tell President Zelensky that he will stand up firmly for Ukraine's sovereignty, territorial integrity, and its aspirations as we go forward."