Trump administration insiders are becoming increasingly concerned that a congressional holdup of key ambassador posts is beginning to interfere with critical American foreign policy efforts, particularly those to rally the international community against North Korea and its contested nuclear program, according to Trump administration insiders and congressional officials familiar with the situation.

Amid a global showdown over North Korea's repeated and increasingly dangerous nuclear tests, Republican leaders in the Senate have declined to hold a vote on several of President Donald Trump's picks for U.S. ambassador, most notably Richard Grenell, the former U.S. spokesman at the United Nations who was tapped in September to serve as the next American ambassador to Germany.

With no indication that Congress will move in the near future to confirm Trump's picks, congressional and administration insiders are expressing growing concerns that the holdup is interfering with U.S. attempts to rein in North Korea's nuclear program, according to multiple sources who spoke to the Washington Free Beacon about the situation.

The State Department has been working to pressure Germany and other countries to pull their ambassadors from Pyongyang as part of a larger effort to isolate North Korea from the international community.

Germany has yet to comply with U.S. requests, leading some to express frustration over the vacant ambassador post in that country and other key nations that could help forward U.S. diplomatic efforts to combat North Korea.

A spokesman for Senate Majority Leader Mitch McConnell (R., Ky.), who has the power to push these key votes along, told the Free Beacon late last week that there is no timeframe for confirming ambassadors such as Grenell, sparking further frustration among some in Congress and the White House.

"We need a U.S. ambassador in Berlin urgently," one lawmaker told the Free Beacon, speaking only on background about the sensitive situation. German Chancellor Angela Merkel "is struggling to form a new government after the collapse of the Jamaica Coalition talks. For anyone who cares about our transatlantic relationship with Europe's number one economy, moving our ambassador to a final vote should be a priority."

Trump supporters on Capitol Hill have begun to quietly express concerns about the holdup and the interference it is causing in U.S. diplomatic efforts.

"This is becoming a bigger problem by the day," said a senior Republican congressional official, also speaking on background. "Without proper representation within some of our greatest international partners, the United States is sabotaging its own influence abroad."

"The administration needs to immediately install an ambassador to Germany, and work to ensure our allies sever diplomatic ties with North Korea," the source said.

State Department spokesperson Heather Nauert emphasized multiple times late last week that Germany and other nations must do more to isolate Pyongyang, though there is little indication Germany has gone as far as the Trump administration would like.

The United States has called "upon countries to do a lot more, which could include kicking out an ambassador," Nauert said on Friday.

Nauert also hinted at frustration in Foggy Bottom over Congress's failure to forward personnel choices for critical regions.

"Is this all moving fast enough? Absolutely not," she told reporters. "We would like it to move faster. The secretary has had conversations internally here and I know he has a lot of conversations with people on the Hill, including a flurry of letters that will go back and forth between our building and also members on Capitol Hill, trying to get people through. So we would like it to move faster. Part of that is not having those selections made, but part of that is the responsibility of the Hill as well to get some of those people moved forward."

Secretary of State Rex Tillerson met with German Foreign Minister Sigmar Gabriel at the State Department last week, but officials said the request to pull its ambassador from North Korea was not discussed.

The diplomatic tension has highlighted the need for President Trump to have his ambassadors in place, sources said.

In addition to the North Korea crisis, Germany is part of the U.S. Visa Waiver program, which has come under scrutiny amid renewed security concerns about potential terrorist exploitation of the program.

"Germany is part of the U.S. Visa Waiver program," said one former senior U.S. national security official who spoke about the situation. "The Germans' enforcement of their immigration policies directly impact U.S. national security."

The Trump administration is said to be increasingly concerned over efforts to stall its picks for key posts.

"When the president gets the people he wants, the machine works," said one Republican foreign policy adviser who is close to the administration. "Look at how the decisions were done on Iran certification and our embassy in Israel. Those went through staff, deputies, principals, then feedback from the president, and then again."

"But overseas we still don't have the president's people implementing his policies and engaged in the policy process," the source explained. "Good work is getting done, but it's below the ambassador level, and in diplomacy there are certain things you need an ambassador appointed by the current president to accomplish. It's well past time for the Senate to clear out the backlog of nominees. "