UNITED NATIONS — The U.S. arms control chief urged the world's nations on Wednesday to hold "a realistic dialogue" about rising global tensions and the challenges that must be overcome to create the conditions for nuclear disarmament.

Undersecretary of State Andrea Thomson told the General Assembly's disarmament committee that this proposal "offers a practical way forward," unlike the U.N. treaty to prohibit nuclear weapons, which she called unrealistic.

"A realistic assessment of the security environment must recognize, regrettably, that we have much work to do to create conditions conducive to nuclear disarmament," Thomson said.

She pointed to high regional tensions in South Asia, the Middle East and elsewhere as well as growing nuclear stockpiles in Asia.

She said Russia and China are modernizing and expanding their nuclear capabilities "and pursuing destructive counter-space weapons at the same time they are becoming increasingly assertive in challenging the existing international order." She said Iran is refusing "to come clean about its past nuclear weapons program" and continues to destabilize the Middle East "with its support for terrorism and militancy."

In addition, Thomson accused Syrian President Bashar Assad's government of using chemical weapons and Russia of using the chemical agent Novichok in an attack in southern England. Assad and Russia deny using chemical weapons.

These challenges "cannot be simply wished away or ignored" by supporters of the treaty to ban nuclear weapons, Thomson said. She said they mistakenly see the accord as "a silver bullet that can jump start nuclear disarmament without addressing the security challenges that cause states to rely on nuclear deterrence or engaging in the difficult work that can produce real reductions in nuclear weapons."

The treaty was adopted in July 2017 by a vote of 122-1 with one abstention and will go into effect after 90 countries formally accept it. According to the U.N., it currently has 19 acceptances, approvals or ratifications.

Thomson said, without elaborating, that the U.S. nuclear stockpile is down by approximately 88 percent from the Cold War peak. She said the U.S. and Russia are continuing to implement the New Start Treaty and met "the central limits" in February, putting their nuclear stockpiles "at their lowest points since the 1950s."

But she said numbers don't tell the whole story.

Thomson said a U.S. paper, "Creating the Conditions for Nuclear Disarmament," submitted at a preparatory meeting in Geneva this spring for the 2020 review conference of the Nuclear Nonproliferation Treaty was not an attempt "to place additional 'conditions' or roadblocks on progress on nuclear disarmament" as some countries thought.

She said that what the U.S. is offering with the paper "is an invitation for all states to join us in a realistic dialogue about the state of the security environment — the world as it is — and how we can shape that environment in a way that makes progress on disarmament possible."