Face to face for over two hours, President Joe Biden and Russia's Vladimir Putin squared off in a secure video call Tuesday as the U.S. president put Moscow on notice that an invasion of Ukraine would bring sanctions and enormous harm to the Russian economy.

With tens of thousands of Russian troops massed on the Ukraine border, the highly anticipated call between the two leaders came amid growing worries by the U.S. and Western allies about Russia's threat to its neighbor.

Putin, for his part, came into the meeting seeking guarantees from Biden that the NATO military alliance will never expand to include Ukraine, which has long sought membership. The Americans and their NATO allies said that request was a non-starter.

There appeared to be no immediate breakthroughs to ease tensions on the Ukraine question, as the U.S. emphasized a need for diplomacy and de-escalation, and issued stern threats to Russia on the consequences of an invasion.

Biden "told President Putin directly that if Russia further invades Ukraine, the United States and our European allies would respond with strong economic measures," U.S. National Security Adviser Jake Sullivan said after the call.

He added that Biden said the U.S. would also "provide additional defensive materiel to the Ukrainians ... and we would fortify our NATO allies on the eastern flank with additional capabilities in response to such an escalation."

That could include additional deployments of U.S. troops to eastern European NATO allies, the adviser said.

A top U.S. envoy, Victoria Nuland, said a Russian invasion of Ukraine also would jeopardize a controversial pipeline between Russia and Germany. She told the Senate Foreign Relations Committee Tuesday that if Russia invaded, "our expectation is that the pipeline will be suspended."

Putin's foreign affairs adviser Yuri Ushakov dismissed the sanctions threat during a conference call with reporters.

"While the U.S. president talked about possible sanctions, our president emphasized what Russia needs," Ushakov said. "Sanctions aren't something new, they have been in place for a long time and will not have any effect."

He described the presidents' video conference as "candid and businesslike," adding that they also exchanged occasional jokes.

In a brief snippet broadcast by Russia state television, the two leaders offered friendly greetings to each other.

"I welcome you, Mr. President," Putin said, speaking with a Russian flag behind him and a video monitor showing Biden in front of him. "Good to see you again!" Biden replied with a chuckle. He noted Putin's absence from the recent Group of 20 summit in Rome - Putin took park by video link because of concerns about COVID-19 - and said, "I hope next time we meet to do it in person."

At the White House, Sullivan said, "It was a useful meeting," allowing Biden to lay out in candid terms where the US stands.

As the U.S. and Russian presidents conferred, Ukraine grew only more anxious about the tens of thousands of Russia troops that have been deployed near its border. Ukrainian officials charged Russia had further escalated the smoldering crisis by sending tanks and snipers to war-torn eastern Ukraine to "provoke return fire" and lay a pretext for a potential invasion.

U.S. intelligence officials have not been able to independently verify that accusation, according to an administration official who spoke on the condition of anonymity to discuss the sensitive matter. But the official said that the White House has directly raised concerns with the Russians about "resorting to their old playbook" by trying to provoke the Ukrainians.

The Kremlin, in a post-call readout, said, "Putin emphasized that it's wrong to put the responsibility on Russia, since it is NATO that has been making dangerous attempts to expand its presence on the Ukrainian territory and has been expanding its military potential near Russian borders."

The Russian leader also proposed to lift all mutual restrictions on diplomatic missions and help normalize other aspects of bilateral relations, the Kremlin said. Sullivan said the leaders would direct their staffs to continue negotiations on that.

The leader-to-leader conversation - Biden speaking from the White House Situation Room, Putin from his residence in Sochi -was one of the most important of Biden's presidency and came at a perilous time. U.S. intelligence officials have determined that Russia has massed 70,000 troops near the Ukraine border and has made preparations for a possible invasion early next year.

Sullivan said the U.S. believes that Putin has not yet made a final decision to invade.

Biden was vice president in 2014 when Russian troops marched into the Black Sea peninsula of Crimea and annexed the territory from Ukraine. Aides say the Crimea episode - one of the darker moments for President Barack Obama on the international stage - looms large as Biden looks at the smoldering current crisis.

Politically in Washington, Republicans are framing this moment as a key test of Biden's leadership on the global stage. Biden vowed as a candidate to reassert American leadership after President Donald Trump's emphasis on an "America first" foreign policy. But Republicans say he's been ineffective in slowing Iran's march toward becoming a nuclear power and has done too little to counter autocratic leaders including China's Xi Jinping and Putin.

"Fellow authoritarians in Beijing and Tehran will be watching how the free world responds," Senate Minority Leader Mitch McConnell said from the Senate floor before the Biden-Putin meeting.

Sullivan said Biden and Putin had a "good discussion on the Iran issue" and called it an area where the two countries could cooperate.

"The more Iran demonstrates a lack of seriousness at the negotiating table," the more there will be a sense of unity among the U.S. and the parties to the 2015 nuclear accord including Russia and the European Union, he said.

Trump, who showed unusual deference to Putin during his presidency, said in a statement that "Vladimir Putin looks at our pathetic surrender in Afghanistan, leaving behind dead Soldiers, American citizens, and $85 billion worth of Military equipment. He then looks at Biden. He is not worried!"

Ahead of the Putin call, Biden on Monday spoke with leaders of the United Kingdom, France, Germany and Italy to coordinate messaging and potential sanctions. He also to spoke with them again following his call to brief them out the outcome. Biden is also expected to speak with Ukrainian President Volodymyr Zelenskyy on Thursday.

Putin apparently sees the current situation as a moment to readjust the power dynamic of the U.S.-Russia relationship, analysts agree.

Beyond Ukraine, there are plenty of other thorny issues on the table, including cyberattacks and human rights. Before the call, Kremlin spokesman Peskov said U.S.-Russian relations are overall in "a rather dire state."

"Russia has never planned to attack anyone," Peskov said. He characterized the Biden-Putin call as a "working conversation during a very difficult period," when "escalation of tensions in Europe is off the scale, extraordinary."

From The Associated Prees