The United States ambassador to Russia is refusing to leave the country after the Kremlin “advised” him to return home following new Biden administration sanctions, two sources briefed on the situation tell Axios.Why it matters: John Sullivan, a respected diplomat who President Biden has, so far, retained from the Trump era, is at the center of one of the most important early tests of Biden’s resolve.Get market news worthy of your time with Axios Markets. Subscribe for free.Russia’s foreign ministry announced Friday it would expel 10 American diplomats and bar current officials, such as Attorney General Merrick Garland, from visiting Russia.But the Russians didn’t expel Sullivan. Instead, the Kremlin summoned him to meet with a top foreign policy official, Yuri Ushakov, who recommended he go back to Washington for consultations with Biden officials.Sullivan’s view, according to people familiar with his thinking, is that if Putin wants him to leave, he’ll have to force him.A State Department spokesperson declined to comment.The big picture: Last week, in response to Russian cyber-espionage and interference into U.S. elections, Biden unveiled a series of sanctions targeting the Russian economy.Biden spoke with President Putin last Tuesday, telling him that sanctions were coming while also exploring the possibility of a summit between the two later this year.In announcing the sanctions, Biden called them “proportionate,” and was careful to note he did not want to kick off a cycle of escalation with Putin.National security adviser Jake Sullivan spoke Monday with Nikolay Patrushev, secretary of the Russian Security Council, discussed a possible presidential summit and “agreed to continue to stay in touch,” according to a White House statement.Flashback: Last month, Biden agreed Putin was a “killer.” The Russian — accused of poisoning some of his enemies — responded by wishing the new president “good health.”Putin also recalled Russia’s U.S. ambassador, Anatoly Antonov, to Moscow for consultations.The diplomatic signal was meant to underscore the Kremlin’s unhappiness with the Biden administration.Between the lines: In response to the new sanctions, Russia didn’t expel Sullivan — or make him a “persona non grata,” “PNG” in diplomatic parlance.”If the Kremlin wants to PNG him, that would be a very escalatory move,” said Mike McFaul, who served as ambassador to Russia under President Obama. ”The last ambassador to be PNG’d from Moscow was George Kennan, by Stalin in 1952.””I applaud Ambassador Sullivan for continuing his job,” he said.The intrigue: Russia is amassing thousands of troops along its border with Ukraine.U.S. officials have been working behind the scenes with allies to warn Russia of the costs of seizing additional territory but have made clear they’re not going to intervene militarily to deter Russia in a non-NATO nation.The bottom line: Biden wants stability and predictability to define his relationships with Putin and Russia, while his administration focuses on China.By ignoring the Kremlin’s suggestion to recall Sullivan, the administration isn’t taking the bait to escalate, while making it clear that Biden gets to decide who his ambassador is, not Putin.Like this article? Get more from Axios and subscribe to Axios Markets for free.