UNITED NATIONS (AP) — The United States is urging countries that have ratified a U.N. treaty to ban nuclear weapons to withdraw their support as the pact nears the 50 ratifications needed to trigger its entry into force, which supporters say could happen this week.

The U.S. letter to signatories, obtained by The Associated Press, says the five original nuclear powers -- the U.S., Russia, China, Britain and France -- and America’s NATO allies “stand unified in our opposition to the potential repercussions” of the treaty.

It says the treaty “turns back the clock on verification and disarmament and is dangerous” to the half-century-old Nuclear Nonproliferation Treaty, considered the cornerstone of global nonproliferation efforts.

“Although we recognize your sovereign right to ratify or accede to the Treaty on the Prohibition of Nuclear Weapons (TPNW), we believe that you have made a strategic error and should withdraw your instrument of ratification or accession,” the letter says.

The treaty requires that all ratifying countries “never under any circumstances ... develop, test, produce, manufacture, otherwise acquire, possess or stockpile nuclear weapons or other nuclear explosive devices.” It also bans any transfer or use of nuclear weapons or nuclear explosive devices — and the threat to use such weapons -- and requires parties to promote the treaty to other countries.

Beatrice Fihn, executive director of the International Campaign to Abolish Nuclear Weapons, the 2017 Nobel Peace Prize-winning coalition whose work helped spearhead the nuclear ban treaty, told The Associated Press Tuesday that several diplomatic sources confirmed that they and other states that ratified the TPNW had been sent letters by the U.S. requesting their withdrawal.

She said the “increasing nervousness, and maybe straightforward panic, with some of the nuclear-armed states and particularly the Trump administration” shows that they “really seem to understand that this is a reality: Nuclear weapons are going to be banned under international law soon.”

Fihn dismissed the nuclear powers’ claim that the treaty interferes with the Nuclear Nonproliferation Treaty as “straightforward lies, to be frank.”

“They have no actual argument to back that up,” she said. “The Nonproliferation Treaty is about preventing the spread of nuclear weapons and eliminating nuclear weapons, and this treaty implements that. There’s no way you can undermine the Nonproliferation Treaty by banning nuclear weapons. It’s the end goal of the Nonproliferation Treaty.”

The NPT sought to prevent the spread of nuclear arms beyond the five original weapons powers. It requires non-nuclear signatory nations to not pursue atomic weapons in exchange for a commitment by the five powers to move toward nuclear disarmament and to guarantee non-nuclear states’ access to peaceful nuclear technology for producing energy.

U.N. Secretary-General Antonio Guterres called the nuclear weapons ban treaty “a very welcome initiative."

“It is clear for me that we will only be entirely safe in relation to nuclear weapons the day where nuclear weapons no longer exist," he said in an interview Wednesday with AP. “We know that it’s not easy. We know that there are many obstacles."

He expressed hope that a number of important initiatives, including U.S.-Russia talks on renewing the New Start Treaty limiting deployed nuclear warheads, missiles and bombers and next year's review conference of the Nuclear Nonproliferation Treaty, “will all converge in the same direction, and the final objective must be to have a world with no nuclear weapons."

“That the Trump administration is pressuring countries to withdraw from a United Nations-backed disarmament treaty is an unprecedented action in international relations,” Fihn said. “That the U.S. goes so far as insisting countries violate their treaty obligations by not promoting the TPNW to other states shows how fearful they are of the treaty’s impact and growing support.”

The treaty was approved by the 193-member U.N. General Assembly on July 7, 2017 by a vote of 122 in favor, the Netherlands opposed, and Singapore abstaining. Among countries voting in favor was Iran. The five nuclear powers and four other countries known or believed to possess nuclear weapons — India, Pakistan, North Korea and Israel — boycotted negotiations and the vote on the treaty, along with many of their allies.

The treaty currently has 47 ratifications and needs 50 ratifications to trigger its entry into force in 90 days.

Fihn said there are about 10 countries that are trying very hard to ratify to get to 50, “and we know that there are a few governments that are working towards Friday as the date. ... We’re not 100 percent it will happen, but hopefully it will.”

Friday has been an unofficial target because it is the eve of United Nations Day on Oct. 24 which marks the anniversary of the entry into force in 1945 of the U.N. Charter. The day has been observed since 1948 and this year is the 75th anniversary of the founding of the U.N.

Fihn stressed that the entry into force of the treaty will be “a really big deal” because it will become part of international law and will be raised in discussions on disarmament, war crimes and weapons.

“And I think that over time pressure will grow on the nuclear-armed states to join the treaty,” she said.