By David Morgan

WASHINGTON (Reuters) - Top national security advisers to President-elect Donald Trump and outgoing President Joe Biden are working "hand in glove" to pose a united front against U.S. adversaries during the presidential transition, Trump adviser Mike Waltz said on Sunday. 

Waltz, a Republican congressman tapped by Trump to serve as his national security adviser after taking office Jan. 20, said he is in discussions with his Biden administration counterpart, Jake Sullivan, as the United States confronts an escalating war in Ukraine and ongoing hostilities in the Middle East. 

"For our adversaries out there that think this is a time of opportunity, that they can play one administration off the other, they're wrong," Waltz, a Trump loyalist and former Army Green Beret, told "Fox News Sunday." 

"We are hand in glove," Waltz added. "We are one team with the United States in this transition."

Trump could face a rocky Senate confirmation process for other top national security picks viewed as inexperienced or otherwise flawed, including Tulsi Gabbard, a former Democratic congresswoman who has been chosen as intelligence chief, and Pete Hegseth, a former Fox News host nominated to head the Defense Department.

Gabbard has implied that Russian President Vladimir Putin had valid grounds for invading Ukraine and stirred controversy by meeting Syrian President Bashar al-Assad in the midst of his bloody crackdown on dissidents in 2017. Hegseth, a military veteran, has been accused of sexual assault, though no criminal charges were filed, and Hegseth has said the encounter was consensual. 

Trump's initial choice for U.S. attorney general, Matt Gaetz, withdrew from consideration last week in the face of Senate opposition over allegations of illicit drug use and sex trafficking.

Despite lawmaker concerns about Trump nominees, Senator John Barrasso told "Fox News Sunday" in a separate interview that Republicans are aiming to confirm most of Trump's cabinet picks quickly so that they would be in place soon after he takes office on Jan. 20.

"It's critical for us in the Senate to make sure that on day one, President Trump has confirmed his national security team," said Barrasso, who will serve as No. 2 Republican when his party takes control of the chamber in January. 

Waltz said the incoming Trump administration is prepared to work with European NATO allies and others to end what he called an escalating conflict in Ukraine, where Russian forces have begun using a new intermediate-range ballistic missile. 

"It is just an absolute meat grinder of people and personnel on that front. It is more like World War One trench warfare," Waltz said, emphasizing the need to begin negotiations.  

"What we need to be discussing is who's at that table, whether it's an agreement, an armistice, how to get both sides to the table, and then what's the framework of a deal," Waltz said. "That's what we'll be working with this administration until January and then beyond."

(Reporting by David Morgan; Editing by Scott Malone)