WASHINGTON (Reuters) - Top Senate Democrats on Monday told President Donald Trump not to make a deal that leaves North Korea with nuclear weapons, and threatened to maintain or toughen sanctions on Pyongyang if that condition is not met.

Senate Minority Leader Chuck Schumer (D-NY) speaks to members of the media during a news conference at the U.S. Capitol in Washington, U.S., May 22, 2018. REUTERS/Leah Millis

Senate Democratic Leader Chuck Schumer and ranking Democrats from national security committees released a letter to Trump laying out demands for any pact, which they said must be permanent.

Trump plans a summit with North Korean leader Kim Jong Un on June 12, the latest twist in the high-stakes diplomacy over U.S. attempts to eliminate Pyongyang’s nuclear arms program.

The Democrats also told Trump to lean hard on Kim’s ally China, to ensure it “will do all it can to help secure an agreement and then insist on strict North Korean compliance with such an agreement.”

Easing sanctions under a deal would likely need approval from Congress which has passed sanctions on North Korea. Since most legislation needs 60 votes to pass the 100-member Senate and Trump’s fellow Republicans hold only 51 seats, that would require Democratic support.

The Democrats’ demands include North Korea dismantling and removing every nuclear, chemical and biological weapon, ending production and enrichment of uranium for weapons purposes and permanently dismantling its nuclear weapons infrastructure.

They said North Korea must also agree to suspend ballistic missile tests and commit to robust inspections.

“Now that the meeting will proceed as planned, we want to make sure that the president’s desire for a deal with North Korea doesn’t saddle the United States, Japan and Korea with a bad deal,” Schumer told a conference call with reporters.

Democrats have been struggling to balance their support for peace with North Korea and frustration with what they see as Trump’s refusal to work with them. On Monday, they said they want Trump to achieve “a lasting and strong agreement,” but he must respect allies and engage closely with lawmakers.

“We just can’t settle for something less than what will ultimately make the peninsula, the region and the world more secure,” said Senator Bob Menendez, the top Foreign Relations Committee Democrat.

Schumer said talks with North Korea are far more perilous than negotiations over Iran’s nuclear program because Tehran did not have nuclear weapons or intercontinental ballistic missiles, unlike Pyongyang.

Trump enraged Democrats and frustrated allies last month by announcing he was pulling out of the Iran deal. Many Republicans said Trump was right to withdraw from the pact reached by Democratic former President Barack Obama because it was not a treaty approved by Congress.