Venezuela was rocked by violent clashes on Tuesday between opposition protesters and security forces still loyal to Nicolas Maduro, the country's socialist president, in a dramatic escalation of the political crisis that has left the country teetering on the edge of violence for months.

The U.S. said that Maduro was prepared to relinquish power and flee the country, but was convinced to stay by Russia. The administration is now threatening Cuba with a full embargo if it does not withdraw support for Maduro and telling the leader's senior advisers they have hours left to abandon him.

The violence began after opposition leaders Juan Guaido -- who the U.S. and 53 other countries back as the legitimate leader -- and Leopoldo Lopez -- freed from house arrest by deserted Venezuelan security forces called for an uprising early Tuesday morning. They said they had gained military backing and would begin the "final phase" of their push to oust Maduro.

As their supporters poured into the streets, government forces met them with tear gas and heavy weaponry. In addition to reports of gunfire, video showed Maduro's armored vehicles rolling after protesters, in some cases trampling some as they attacked with Molotov cocktails and baseball bats. At least 50 people have been reported injured, according to the Associated Press, including one by gunfire.

President Donald Trump voiced support for "the People of Venezuela and their Freedom" in a tweet and threatened Cuba with a "full and complete embargo" and "highest-level sanctions" unless it withdrew its military support for Maduro.

Then, Secretary of State Mike Pompeo told CNN that Maduro was in the final stages of exiting, that he had a plane ready on the tarmac to depart for Cuba -- when a last-minute intervention by Russia convinced him to stay. When asked what his message to Maduro was, Pompeo said flatly, "Fire up the plane."

ABC News could not confirm that detail and Pompeo declined to say how the U.S. knew that.

But earlier in the day, the State Department Special Envoy for Venezuela Eliot Abrams told reporters that three senior Maduro officials had been in negotiations with Guaido's government to abandon the socialist leader. Defense Minister Vladimir Padrino Lopez, National Guard chief Ivan Hernandez Dala and Supreme Tribunal of Justice President Maikel Moreno were in talks to recognize Guaido as the legitimate president, he said, but they still hadn't done so by Tuesday evening.

National security adviser John Bolton issued an ultimatum to them: "Your time is up. This is your last chance. Accept Interim President Guaido’s amnesty, protect the Constitution, and remove Maduro, and we will take you off our sanctions list. Stay with Maduro, and go down with the ship," he said in a tweet.

The U.S. did not have advanced knowledge of Guaido and Lopez's plans, according to Abrams, but he told reporters the moment was "not done out of the blue," but part of the "long process to restore democracy" in the country. Abrams and Guaido exchanged messages Tuesday, according to the U.S. diplomat, who described the opposition figure as "buoyant and determined."

But so far, security forces seemed to maintain their loyalty to Maduro, with Defense Minister Padrino again reiterating his support and saying Guaido is responsible for any violence or deaths. Dozens of security forces engaged opposition protesters, reportedly firing rubber bullets, deploying tear gas and even using armored vehicles to intimidate and at least at one point run over demonstrators.

"The situation on the ground remains confused, and we have conflicting reports. We urge the Venezuelan military to stand up for the nation and for the constitution and stand behind the people of Venezuela. They will be judged by their actions today," Abrams said.

While Maduro said in a tweet earlier in the day that he had "total loyalty" from military leaders, he has not been seen in public or on state television since the day's protests erupted. Still, his supporters blasted Guaido as a U.S. puppet pushing a coup.

Guaido's representative to Washington fought back.

"This is not a military coup," Ambassador Carlos Vecchio told reporters. "This is a constitutional process led by the Venezuelan people under the leadership of a civilian, the interim president of Venezuela Juan Guaido."

Describing the current situation as a "very delicate moment," Bolton also insisted the administration does not view the ongoing uprising as a coup.

"This is clearly not a coup, we recognize Juan Guaido as the legitimate interim-president of Venezuela," he said.

Bolton repeatedly declined to say whether the U.S. would consider taking any military action to support the opposition.

"All options remain on the table, I’m simply not going to be more specific than that," he said, even as he also emphasized the administration’s desire for a "peaceful transfer of power." Bolton said the president is monitoring the developments on the ground in Venezuela "minute by minute," while Pompeo warned any arrest of Guaido would be seen as a "major escalation" by the U.S.

Support for Guaido and opposition to Maduro has been a rare bipartisan issue in Washington. Democratic lawmakers like Sen. Bob Menendez, D-N.J., and Dick Durbin, D-Ill., again voiced their support, saying they were briefed by Bolton on Tuesday morning.

Maduro has faced protests for months over his economic mismanagement, consolidation of power and crackdown on political opposition. The opposition-controlled National Assembly voted to declare Guaido interim president in January, winning the backing of the U.S., several neighbors like Colombia and Brazil, and many European countries. But Maduro, backed by allies like Russia and Cuba, has maintained his control on power, in large part by sustaining the support of the military chiefs.

ABC News's Aicha Hammar contributed to this report.