The US has accused China of dramatically expanding its nuclear arsenal, while doubling down on claims that Beijing had conducted secret nuclear tests.

Washington said the lapsing of New Start – the last treaty between top nuclear powers the US and Russia – earlier this month presented the possibility of striking a “better agreement” that included Beijing.

Christopher Yeaw, the US assistant secretary of state for arms control and non-proliferation, told the Conference on Disarmament in Geneva that New Start had been seriously flawed and “did not account for the unprecedented, deliberate, rapid and opaque nuclear weapons buildup by China”.

“Despite its claims to the contrary, China has deliberately and without constraint, massively expanded its nuclear arsenal without transparency or any indication of China’s intent or end point,” he said.

The Chinese ambassador Shen Jian told the conference that his country “firmly opposes the constant distortion and smearing of its nuclear policy by certain countries”, insisting that Beijing would not “engage in any nuclear arms race, with any country”.

Russia and the US have more than 5,000 nuclear weapons, according to International Campaign to Abolish Nuclear Weapons (Ican), the Nobel peace prize-winning campaign group.

New Start, which expired on 5 February, restricted the US and Russia to 1,550 deployed nuclear warheads each – a number Washington claims Russia has passed and China is fast approaching.

“Beijing is on track to have the fissile material necessary for more than 1,000 nuclear warheads by 2030,” Yeaw said.

Shen insisted that “China’s nuclear arsenal is not in the same league as the countries possessing the largest nuclear arsenals”.

“It is not fair, reasonable or realistic to expect China to participate in the so-called trilateral talks,” he said.

However, a US state department source told AFP that a “preparatory” meeting had taken place with a Chinese delegation in Washington the day after New Start expired, and a more “substantive” meeting was scheduled in Geneva for Tuesday.

Yeaw said that New Start’s limits on warheads and launchers were “no longer relevant”, given Russia’s alleged violation of the treaty. He also accused Moscow of helping “boost Beijing’s capacity to increase its arsenal size”.

The US is not “walking away from or ignoring arms control”, Yeaw said on Monday, adding: “Our goal, is a better agreement toward a world with fewer nuclear weapons.”

New Start’s expiration marks the first time in decades that there is no treaty to curtail the positioning of the planet’s most destructive weapons, prompting fears of a fresh arms race.

Yeaw, who last week indicated that Trump was serious about returning to testing on an “equal basis”, doubled down on accusations that China had conducted secret nuclear tests. He provided more details on a low-yield test that Washington has said Beijing conducted in 2020 and accused China of preparing more explosions with larger yields.

Yeaw told the conference that data gathered in nearby Kazakhstan showed that China conducted a 2.75-magnitude explosion underground on 22 June 2020.

“The estimated yield of the event was a 10-tonne nuclear explosion, or five tonnes conventional equivalent, which assumes the explosion was fully coupled in hard rock below the water table,” he said.

Trump said last October that the US planned to resume nuclear testing to match the alleged secret explosions by China.

Shen called accusations that China has conducted a test “groundless”, and accused Washington of using its claims “as a pretext” to resume nuclear testing.

In a recent report, the Center for Strategic and International Studies did not find conclusive evidence of an explosion, saying satellite imagery did not show unusual activity at Lop Nur, China’s historic testing site in the western region of Xinjiang.