Two months after President Trump shook hands with North Korean leader Kim Jong Un at the Korean demilitarized zone, his administration remains stymied in its efforts to coax Pyongyang back to the negotiating table, leading to mounting frustrations that time is running out while Kim has strengthened his position.

In a new letter to Trump on Friday, leading Senate Democrats are calling the North’s recent short-range-missile tests “a significant step backwards” and demanding that the United States hold Kim’s regime accountable for actions that “clearly contravene” U.N. Security Council resolutions, according to a copy obtained by The Washington Post.

A leading Washington think tank concluded in a public assessment published Friday that an undisclosed North Korean operating base in Kumchon-ni houses medium-range ballistic missiles capable of striking southern Japan as well as the outskirts of Tokyo.

And the State Department, whose efforts to engage North Korean diplomats have floundered, is weighing new steps to publicly pressure the Kim regime during the U.N. General Assembly in New York this month, said officials with knowledge of the internal deliberations.

The developments stand in sharp contrast to Trump’s efforts in recent weeks to play down friction with the North in an effort to maintain a personal relationship with Kim that has driven his un­or­tho­dox strategy to force one of the world’s most brutal and dangerous regimes to relinquish its nuclear arsenal.

“The president is increasingly isolated within his own government on this policy,” said Victor Cha, Korea chair at the Center for Strategic and International Studies (CSIS), whose program published the assessment of the Kumchon-ni facility.

Cha noted Trump said last month that he had received a letter from Kim suggesting talks would resume after the completion of annual joint military exercises between the United States and South Korea, which ended two weeks ago.

“I sense a lot of frustration that nothing is going on,” Cha said of the president’s aides. “They promised in August that these things would happen.”

White House officials declined to comment.

Kim has unsettled East Asia this summer with tests of short-range missiles that Japanese Prime Minister Shinzo Abe called a clear violation of U.N. Security Council resolutions. But Trump has consistently maintained that the tests do not violate a personal pledge Kim made during their first summit in Singapore last year to refrain from testing intercontinental ballistic missiles or nuclear weapons.

Despite Trump’s unwillingness to criticize Kim, U.S. officials have been rebuffed time and again in their overtures to Kim’s underlings since the two leaders met at the Korean DMZ in late June, according to people briefed on the matter.

Secretary of State Mike Pompeo’s hopes for a meeting with Foreign Minister Ri Yong Ho at the U.N. General Assembly were dashed when the North announced this week it would not participate in the international gathering, a shift from the past three years, when high-level delegations attended.

Two weeks ago, Ri insulted Pompeo, whom Pyongyang views as a hard-liner, by calling him “poisonous” to diplomacy.

During a private briefing on Capitol Hill last week, a senior Trump administration official told congressional staffers that U.S. officials were continuing to reach out to Pyongyang but had heard nothing back, according to a person familiar with the discussion.

“As we have indicated, we are prepared to engage in negotiations as soon as we hear from our counterparts in North Korea,” a State Department official told The Post, when asked for a response regarding Ri’s criticism of Pompeo. The official spoke on the condition of anonymity due to the sensitivity of the situation.

Jean H. Lee, a Korea expert at the Wilson Center who spent five years as the Associated Press bureau chief in Pyongyang, said the North Koreans have signaled uncertainty over how to proceed ahead of an end-of-the-year deadline, set by Kim, to walk away from bilateral engagement if there is no deal.

Kim tested Trump in launching the short-range missiles. Now, Lee said, the North Korean leader could be contemplating whether to ratchet up the stakes by “doing something more provocative.”

Trump has consistently framed his North Korea diplomacy as a success, touting Pyongyang’s nuclear test moratorium and emphasizing that his administration has maintained tough economic sanctions on the Kim regime.

But experts said Kim has used the period of engagement to accelerate development on other aspects of his missile programs. The New York Times reported this week that Kim’s regime has greatly improved the “range and maneuverability” of its weapons systems.

The new assessment from CSIS’s “Beyond Parallel” program provides significant insights into one of the North’s estimated 20 ballistic missile operations that the Kim regime has not disclosed to the public.

The facility at Kumchon-ni is located less than 50 miles north of the DMZ and is equipped with Hwasong-9 medium-range ballistic missiles capable of striking throughout South Korea in addition to southern Japan, according to the report.

The findings come as Japan and South Korea have ended an intelligence-sharing agreement amid a worsening diplomatic row that has roots in historical grievances dating to Japan’s wartime occupation of the Korean Peninsula.

“North Korea’s ballistic threat is significant,” said Joseph S. Bermudez Jr., a CSIS analyst who compiled the report based on commercial satellite imagery, declassified government reports and interviews. “It is growing, and it is something that needs to be addressed not only for the United States but for allies in Japan and South Korea.”

In their letter to Trump, eight Democratic senators, including Minority Leader Charles E. Schumer (N.Y.), Richard J. Durbin (Ill.) and Robert Menendez (N.J.), call on him to use the U.N. General Assembly to push for accountability measures on the North’s missile activities while also seeking to “establish the sort of regular working-level negotiations necessary for diplomacy to succeed.”

Yet it’s not clear how such talks would resume. Trump’s second summit with Kim in Hanoi in February collapsed after Kim offered to close a major nuclear processing facility in exchange for relief from economic sanctions, but Trump demanded that the North agree to relinquish its entire nuclear and ballistic missile programs.

Since then, neither side has signaled a willingness to offer concessions. The standoff comes amid reports that Stephen Biegun, the lead U.S. negotiator with North Korea, would be in the running for a promotion to deputy secretary of state if Trump taps John Sullivan as the new ambassador to Russia. In that event, Biegun probably would stay involved in the talks but appoint a deputy to manage the negotiations on a day-to-day basis.

U.S. officials have cautioned that nothing has been decided.

Michael Green, who served as a top Asia policy official in the George W. Bush administration, said Japanese and South Korean diplomats have grown increasingly pessimistic about the future of the negotiations.

North Korea’s success in developing its weapons capabilities could provide Kim with increased leverage in the negotiations and the ability to “escalate the situation when the time is right to test again,” Green said. “Everyone I talk to on both sides think that’s where it’s headed — toward another crisis.”